Welcome to BYC!
Rightly so on the mad chick.. there's safety in numbers.. If she isn't lethargic or being pushed over and trampled.. get her back with the others..
What you describe so far, being smaller.. just sounds possibly like different genetics.
But Wyorp Rock is correct that Coccidiosis is the most likely thing to impact chicks this age... usually indicated by being not interested in food or water, hanging out under the heat lamp MORE, standing around puffed up with head pulled in towards chest.
The picture looks like a chunky little chick!
Hope your just being an overly cautious mum and they all continue to thrive!
Consider starting a new thread on just her situation if you're still concerned.. so maybe it won't be muddied by the old information here.. each situation is unique.. even when symptoms appear similar, in my expereince.. Sometimes heavy breathing can be a symptom one is on their way out.. other times.. respiratory illness, and yet again sometimes.. just an indicator they got temporarily too warm, & panting is ONE way they cool off.
