Size difference between TE's and silicone implants

Hi everybody

I'm currently going through the filling process, and before my reconstruction I was flat for a year. I had a prophylactic surgery, and before that I had breast cosmetic implants, Mentor style "CPG 312", 255 cc. Implant width was 11,5 cm and height 10,5 cm. I had about 175 cc of my own breast tissue at that time. 

When the implants were taken out I saved them as "souvenirs". During the year when I was flat I tried them on just for fun, since I was thinking about what the new size would be, etc. but it was obvious that 255 cc was really small, it was hardly even noticeable. 

The odd thing is that now when I'm in the filling process, I currently have 250 cc, which is approximately the same volumes as my old implants. But it looks much better; the expanders seem to be both much wider and sort of have a "better" shape. And the size is good to - I will go a bit bigger but 250 is absolutely ok too.

My TE's are Mentor CPX 4, Low Height (width about 12,7 cm and height about 9,4 cm)

What I don't understand is how 250 cc Tissue Expanders can look so much wider and generally bigger than the silicone implants I had. I've studied the size charts and it doesn't seem like there is a very big difference between the silicones I had and the TE's I have now.  But despite the really small differences in the implants, there is a big visible difference. Do I have that thick breast muscles or what could the reason be?