My two boys are big: about 1410 grams for Badger and 1305 grams for Tufty. Badger is 10.5 to 12 inches long, nose to rump, depending on his position when I measure him. (Abyssinians have this 'crouch' position they go into, shortening their body cutely like a cat.) Tufty is about 8.5-10 inches long. They are now 25 months and 21 months old respectively, but they were not that much smaller when I got them at 9 months and 5 months. Tufty grew more; at 5 months old it turned out he had another 100 grams to go before reaching his full adult weight.
It is hard to find kitchen scales big enough for them. My last two scales were not really big enough. I'm still searching for a good digital one which they can walk on to, but does not have a display that their fat little bodies cover. (that's the problem with the last scale I got!lol)
I did not know they were particularly big for piggies until I kept hearing from people who saw them that they were "huge for guinea pigs". I wonder sometimes if many people are just more familiar with those large breeds of hamster that are so popular, and think that piggies should be about that size. To me, my boys are quite normal sized, in fact the optimal size (in my current biased view). They are like small fluffy bunnies. (but much sweeter and more caressable). Yes, I think Badger is not much smaller than some of those Maltese or other 'designer' dogs.
Weigh your mom's dog and see how close your piggies get to that weight! Yours are sows so they will probably not get as big as my boys.