We just released Sex Sim update 1.0g, and one of the things it includes is the ability to have bigger girls.
We'll be putting up some new characters in the store with these new body styles but they are ready to use right now by simply altering the XML files for custom characters.
To use the new body styles, open up the character XML file and look for the "<Body>" tag. It will either have a 1 or 2.
To use the "thick" body type, change the number to a 3, and to use the "big" body type, change it to a 4.
The larger body styles look better with larger breasts, so you probably want to change those values as well. I've found that the following settings work well for the "big" shape:
<BoobSize_LX>1.1</BoobSize_LX>
<BoobSize_LY>1.2</BoobSize_LY>
<BoobSize_LZ>1.1</BoobSize_LZ>
<BoobSize_RX>1.1</BoobSize_RX>
<BoobSize_RY>1.2</BoobSize_RY>
<BoobSize_RZ>1.1</BoobSize_RZ>
Your mileage may vary depending on the other values you have in your custom characters, but these new larger body styles are designed for larger breasts so have fun.