Totoro is weighing in at about 10 pounds, which is a not unreasonable weight for a tom coming up 9 months.
The surprise is that Sakura is 9 pounds. Put them together, and she looks smaller than her brother, but weight-wise, there doesn't seem to be much in it. I think it's down to the colouration: he's a black-flecked brown while she's fawn, and I think it's that dark coloured things look larger than their light-coloured equivalents.
They're also both mostly muscle. She managed to push open the bedroom door yesterday morning, despite it supposedly being shut.