Small poops can occur if he is not drinking enough water or as an indicator of the onset of stasis. You could dip his veggies in water to help keep him hydrated, or give him some simethicone (baby gas drops but not Gas-X) if he has a little gas. It’s important that he eat more hay because buns get most of their fiber from hay, as opposed to pellets. Maybe try a different brand/type of hay, or a different hay feeder design?

After my Bun Bun came home from the vet, I had to give her a shot of Penicillin every day. She seemed totally fine otherwise, but her poops were getting small. I was told that it might just be a side effect of the medication. Then one morning she had a bunch of messy poops everywhere and she later passed that day

Even thought not totally the same situation, I don’t think it would hurt to get your bunny checked out just in case

If he hates hay, you really need to combat that first and foremost. Try different types, brands and different methods of offering it.

Smaller poops aren’t necessarily a sign of illness but if he’s not as enthusiastic about food (his pellets) and isn’t drinking as normal, it’s definitely something to keep an eye on. It sounds like it could be dehydration so make sure you offer his veggies really wet to increase his fluid intake. Hopefully then his poops might go back to normal but if he stops pooping altogether, obviously go to an emergency vet.

If he’s off of his pellets, and he doesn’t eat hay, I would be concerned that he may have tooth spurs. Eating hay is the primary way they wear their teeth down, without that it’s possible his are overgrowing a little. This may be the reason he’s gone off of his pellets, the spurs may hurt while eating and usually hard foods are the first things they stop eating if they have tooth pain.

You can give simethicone (gas drops) as a precautionary measure, it doesn’t hurt to give even if it’s not gas, but I think it would be worth getting his teeth checked out.