Remember pwys doesn't mean weight:
pwys = a pound (Imperial unit of weight)
pwysau = pounds (plural of above); weight (in general)
Interesting. It did - see GPC, but clearly doesn't now. I wonder when and why it changed to pwysau
To gain weight = ennill pwysau?
You can use magu pwysau lit. “to nurture weight” to mean the same thing too.