I fear that I disagree with all the ‘conventional’ advice that’s out there concerning the feeding of bettas.
I’ll qualify that by saying that yes, some bettas (& many kinds of other fish as well) can be very greedy feeders and being fat is as bad for fish as for any other creature. There need to be reasonable limits.
But there is sooooo much advice out there about bettas specifically being very greedy and having a stomach that is very tiny and I also strongly disagree with that size-of-an-eye mantra because it makes no sense to me.
A stomach is not a jar or a can with finite dimensions. Stomachs can stretch - sometimes quite dramatically and wild fish generally will gorge when they find food in abundance because abundance is typically temporary.
For fish it’s a case of ‘get as much as you can while you can’ because tomorrow, who knows if you’ll find dinner.
I feed bettas the same way I feed most fish, with a limit on how much time they have to eat the food, if I’m feeding dry food.
For dry food they’ll get as many pellets as they care to eat during a period of time - 15-30 minutes for most of them and maybe 5 minutes for one that always seems to inhale food as fast as it can. After a week of doing that, I have a very good idea of how much a given betta is going to consume and I give that much, mainly to avoid wasting food.
None of my fish have ever become obese or fat with this feeding regime. And none have ever been too thin (except at purchase perhaps), let alone emaciated.
Sadly, I have seen far too many pictures of terribly emaciated bettas whose owners surely must have had the best of intentions but were starving them to death slowly due to fear of overfeeding.
When I feed live foods, which I greatly prefer to do for bettas, they have unlimited access to the live food at all times. They catch and eat what they want when they want.
Mainly it’s scuds & daphnids, but I offer worms when I can along with fruit flies & their larvae,
brine shrimp & anything else I can manage to provide. Foods that won’t live too long in water are given with that in mind so there aren’t things like a pile of dead fly larvae lurking later on.
Live fed fish don’t get fat either and I believe that hunting for their dinner provides the best enrichment of their environment that’s possible.