that is correct, they poop sizes will differ throughout the day. if they were small for a full day or two, i would say make a vet apt, but like us, if they just ate not too long ago, their poops will be bigger, but if they haven't eaten for awhile, their poops will be smaller. so it will be completely normal to always notice some bigger ones and some smaller ones.
but like i said, if they are all small at any time, something may be wrong.