This requires a fine-tune that already knows NSFW motions - this is just an enhancer - I recommend 10eros

Stomach distention from penetration is a real thing, the anime version of it are generally greatly exaggerated in a way that I find off-putting, so I tried to capture a realistic version of belly bulge.

It's rather finicky. Sometimes it actually does the opposite where it sucks in the belly when penetrating and you will have to play with the seed or prompt a bit. It's sometimes a rather subtle effect, but noticeable when comparing side by side with and without it, but also sometimes it's pretty significant and unrealistic. In general though it avoids the 'anime style' of belly bulge though.

10eros base has better nsfw motion without this lora so you might want to try on a lower weight or with more advanced prompting.