Looking closer during water time today I think the bigger of the 2 actually may have a sipped tendon in one of its legs. I notice one of his ankles is far more inflamed than the other. I also notice that what looks like the tendon that would normally run along the back of the hock joint seems to go to the side of it. This is on his worse of 2 legs. The other is standing higher on its legs, and while still a bit pigeon toed seems to be able to get around. That one trips occasionally, but is mostly ambulatory.
I may be a bit off on my "1 lb per week" estimate, but when I weighed them at 2 weeks they were both just shy of about 2 lbs. I haven't weighed them in a couple weeks now but will try and get weight to see.
Too bad you didn't notice this when younger you may have been able to fix it. I doubt now there is any chance of it going back in place. Does it seems to be in pain?