yes, i have had the same problem. I have read several places that the bigger/older birds will bother the younger/smaller birds--it is the 'pecking order' you hear about. If they don't hurt the birds, but make them eat in 'at the back of the line' then just make sure they get fed. If they won't ever get as big as the rest of your flock, you may have to make a special spot just for them.
Mine are all bantums and will eventually get to be the same size, but the two newbies will always be the 'new kids' and may be treated differently.
We just have to watch them carefully. Sometimes things level off in a week and sometimes they dont. If they don't well, then, it's more fencing material! ha!