Ok. Normally meat chicks get a meat bird feed (crumble) from hatch to processing that is often approx 22%. Since they are bred to create muscle very efficiently/quickly from feed, that is where people get the huge carcass at 8 weeks. While yours look big, they do not look as big as mine do at 8...
Its great that they are moving around well. Usually 8 weeks is a very typical age to butcher. Some people wait longer, some a little less time. They look to be a good size now - have you picked them up to assess their body mass, and size of the breast?