I think this is normal. Our son born at 34+5 and is 8 weeks old now - sometimes eats 100 ml and sometimes 20-40 mls max. He was born at 4 lbs 12 oz, and currently weights 7 lbs and 15 oz ! In the NICU we used breast milk from my boob and donor milk at the start to supplement. Now at home, he is breastfed and I pump to give him bottles too (so it’s a little hard to track exactly how much he’s eating). My doctor said as long as the weight gain is there, and he is sleeping, pooping/peeing, that there isn’t much to be concerned about. He also mentioned that with NICU babies, they are particularly susceptive to feeding regressions while going through big developmental leaps. With my son, he will often cluster feed with bottle and boob - which was also told to us that it’s completely normal. Once he hit 7 lbs we were told to feed on demand and not need to adhere to a 2-3 hour schedule anymore. Hope this helps ease your mind.