Sounds pretty normal to me, most bitches during the last couple of weeks of pregnancy will put on an enormous amount of weight, some will balloon up so much that walking, sitting can become a problem, my bitch a few years ago with her 2nd litter, was so huge in the last 2 weeks I was actually contemplating a C-section for her as with a week to go still I felt the weight on her legs was too much, but after talking to my vet, all was well and fine and as soon as the pups were born she was lovely and slim again.
As long as your bitch is not overweight at the beginning of pregnancy, which yours does not sound to be i.e you could feel her ribs, (as an overweight bitch once the pups are born would find it difficult to clean herself properly and move round to clean her pups) then the weight gain is a mixture of water retention and the pups last growth spurts, it does not necessarily mean a large litter, though in my case it was.

All sounds very normal to me, but size and weight of the pups does not necessarily go with the bitches weight gain, (you will have to wait and see

) glad that you had her vet checked, just relax now and good luck with the whelping.