i dont know if i need to up the prey size or what....

So my dumerils boa no matter what is always firm and never looks deflated.. My arabesque and even my Guyana at times while just laying there does not look square. they look almost like a trapezoid. like they are deflated. my Guyana is over 500 grams finally after being severly sick and almost lost him. I am feeding him once a week with 30-40 gram rats and my arabuesque is the same way. I have not weighed her but I feed her once a week and her rats are around 40-50 grams and she is in shed right now. she just looks deflated. they are both in great health just don't know if I need to up the prey size. both are around 4 foot. I see some people feeding medium rats at that size so I don't know if my two are being under fed or if I should keep them at that feeding schedule. I don't want a fat boa that is why I am doing it like I am.