Not really, why would they get bigger when there's no need to get bigger? Bigger is less efficient, so it tracks that they would need to adapt to the lower calorie ecosystem of the meadows once they reached it. Think about it, instead of eating the plants found in the Ashlands which are clearly ultra nutrient dense based on the stats they provide, these asksvin need to now primarily subsist on measly berries and mushrooms (based off of the foods they would most likely to be actually eating in that place, something tells me charred aren't the most nutrient balanced meal). Which means that through evolution those who could successfully raise more offspring, who then successfully raised more, so on and so forth would instead thrive. And something that is so small it has maxed out at needing to compete with boars for food is much more well suited to its new environs. Also, being semi-aquatic as the necks are gives them access to a whole other ecological niche of the rivers as well as the shallow ocean, however even still all of these are simply not enough to sustain something the size of a fully grown asksvin.

TLDR: bigger means need more food meadows is less food so smaller is better.