Thanks all. Yes, Bijou is the big boy (16 lbs) on the left and Mika (8 lbs) on the right. They actually are full brother and sister although from different litters. They have the same mom and dad kitties.
Mika was the runt of the litter and was born with the voice defect, i.e., she can only make little huffing noises but she's very good at getting herself heard.
I also see differences in their faces, i.e., Bijou has a larger muzzle and looks more masculine whereas Mika has a feminine daintiness about her. (I call her Nanny's Princess.)
They also look darker in the indoor pictures than they really are.