I had the same issue with one of my buff orpingtons…I actually named the chick "Lucky" because I thought she might be a "he" because she was the extra one thrown into my chick order.
After she got over her aspergilliosis (I was obsessed with getting her better- one chick did not make it) she remained runty… I thought she might be a bantam too!! My husband was convinced she was going to die because she was SO small….he thought she would get trampled. He still says she's a little "special". I was determined that she would live.
The differences were: I noticed that she got way more vocal than the other chicks- she peeped more- almost to say "HEY don't step on me!" because she was so little. She remained very vocal right up until recently.
She also got really fast at stealing food from the others…she was so little she could run right under them and take what they were just eating.
I made it a point to stick her on the little perches often so she learned fast. When I started taking them outside, they would all be under an enclosure (the wire top to a rabbit cage) but I would let her stay outside the enclosure to scratch around and sun herself without getting stepped on and stuff. This worked well with her.( Obviously I was outside with them at the time). She LOVES the sun. The moment there is a sunny spot, she flops on her side and gets all fluffy.
I also made it a point to ensure she always got a little "extra"…scrambled eggs, garlic, electrolytes, etc. All the obsessing paid off...
Little Lucky is the one in the back…she is 3 weeks old in these pics
Not a great pic- but at 8 weeks she is catching up (she's on the right)
This pic was 1 week ago...She is the one in the front- still a little behind my other buff orpington…but size-wise, they are they same.
I bet your little chick will catch up in size- it may take until week 10 or 11…. =)