Ever since vikings began to live with the dragons, Astrid was intrigued about their power... ferocity combined with beauty, and dreamed of what it is like to... make love with a dragon.
When she got older, she tried to do it with a Terrible Terror, but unfortunately it was too small to make it pleasurable, and so, she forgot about this weird desire of hers.
But one day she had a dream. A vivid, lucid dream where she was having sex with Toothless, who was passionately licking and caressing her with his tongue and paws in the process. Her bed was totally wet when she woke up, disappointed that it was just a dream, but determined to try it one day.
Sadly, when she finally got to see Toothless' erect member, as he was sleeping on his stone bed in Hiccup's house and seemingly having a wet dream too, she was sure that it won't fit into her, not without troubles at least. One day it changed though...
Astrid was visiting one of the far inhibited islands in one of their search expeditions, and couldn't miss the local traders. One of them was pretty unusual though - she was selling what she claimed to be "magic potions". The assortment ranged from sickness-curing ones to love-inducing, transformation and even size changing potions, which got Astrid's attention. She instantly remembered about her dream about having sex with Toothless, and thought that if these potions were real, they could help her with it. And if not, she will return and burn down the fraudster's business. Maybe.
There were lots of size-changing potions to choose from, and Astrid decided to buy several slow-shrinking ones, some immediate shrinkage potions, some slow and immediate growth ones, and also some protection potions, just in case something goes wrong.
For a Berkian, the price wasn't high, and she happily hurried back to Stormfly, imagining her dream finally coming true.
Having arrived to Berk, Astrid locked her room and began to read the instructions of the potions. She decided to go with the slow shrinking one first, to not make Toothless too small too fast accidentally, and estimated the dose she would need to shrink him down to... compatible size.
Knowing that Hiccup is away from home until tomorrow's morning, and Toothless should be alone in his house, she ran there with some potion vials on her belt, excited for what's to come.
"Toothless, I am home!" - she cried to the black dragon, who crooned in response and could be heard running downstairs to meet her. "Good boy!" - Astrid cheered him, petting his head and scratching his chin. "I have a present for you. Wait a bit." - she added, as she proceeded to get a bowl and fill it with milk, pouring a dose of the potion there, which, she hoped, would be a just right amount.
"Your favorite milk with extra-flavor, Toothless! Here, lap it up." - she placed the bowl on the floor near the excited dragon, who immediately began to lick it, as Astrid sat on the nearest couch and started to wait for the result, smiling.
In just a few moments, Toothless could be seen getting smaller. It was barely noticeable at first, but accelerated, and the large dragon was visibly diminishing in size, to Astrid's joy.
Toothless was happily lapping up the milk, as he suddenly noticed that the bowl seemed larger to him, and so did Astrid, sitting on the couch in front of him, and everything else around. He stopped lapping up the milk and questionably looked at Astrid, as if asking "what's going on?".
"Don't worry, Toothless. I had to make you a bit smaller to make what I planned possible. You will only shrink to a little more than half your size, then we will... hm... mate with you? And I will return you to normal. It won't take longer than a few hours, I promise."
Toothless's eyes went a bit wider as he heard the mating word, but he suspected that this female viking wants to do it with him anyways. What surprised him more was the fact that he already shrunk to about half his size, from the looks of it, and still was visibly getting smaller by the second. Astrid noticed it too, got up and walked closer to him, worrying about the shrinking night fury, realizing that she miscalculated the dose. She was disappointed at first, but seeing the dragon getting smaller and smaller by her feet made her feel... horny.
Toothless got to Astrid's legs and reached up, placing his front paws on her thighs. It was now the highest part of her body that he could reach this way, and still his paws were sliding lower and lower, to her knees, and then below them. He trusted his best friend's wife, but seeing her beginning to undress, starting from upper body, he couldn't help beginning to worry a bit.
Astrid got her shirt off, revealing her shapely medium-sized breasts, and then proceeded to strip her skirt and pants off, revealing her vagina to Toothless, who now stood just a bit higher than her ankles, having placed his front paws back on the floor. She made sure that her clothes do not fall on the tiny dragon, as she wanted to keep seeing him shrinking.
The dwindling night fury was in awe, as he looked up at the ever-growing legs of the beautiful viking woman before him, her vagina, glistening from wetness, breasts, with now erect nipples, and her grinning, pleasured face above it all, obstructed by the breasts a bit. Astrid positioned her feet on the either side of the now tiny reptile, as he reached the height of her ankle, standing on all fours. She knew she may have to give him the growth potion soon, if he doesn't stop shrinking, but wanted to let her desire control her now, while Toothless was still big enough to be noticed.
Toothless looked around himself, seeing the Astrid's feet now higher than he is tall, and something strange clicked in his mind... he liked being towered over by the giantess woman. His penis began to get out of its sheath, as he adored the view more and more, and Astrid only added more fuel into his desire as she placed her toes right in front of him, with a gust of wind.
Toothless noticed he has stopped shrinking in size, now only as high as Astrid's big toe at most, being only around 3 or 4 inches long, and this fact made his penis erect to its maximum potential.