LARGE slow chicken

I only have 4 hens, two are Reds and two are some breed of black chickens. One of my blacks is quite large, I would say the others might be 8 or 10 pounds, but Betsy is I bet 13 or 14 pounds....(so say an extra 50% heavier than the other girls).

Betsy tends to be slow, she will stay in one place while the others wander all over. Betsy does move around the yard, only at a much more leisurely pace! If I throw birdseed at their feet, the other 3 attack it as if they are ravenous, Betsy stands there and looks at it. (If I throw bread at their feet, all hell breaks loose, including Betsy who will push and shove with the rest of them to get to it).

A few weeks ago Betsy was very lethargic, so I took her to the vet who diagnosed a bound crop. She seems to be over that now (no more funny neck moving going on), but she just seems off. Is it because she's just so big? I am just asking for opinions! Also, I leave food out at night in their coop, and maybe I shouldn't, because I often catch Betsy standing in the coop alone eating the feed....

Appreciate any ideas!