If you open the Info.plist file in the Sweet Home 3D.app folder, you'll see that it contains the following text:
meaning the Mac OS X version of Sweet Home 3D installer (and the Linux version too) already requires 1024 MB at max.

In version 2.1, I decreased max memory to 512 MB only for Windows installer and Java Web Start version, because the could not create the Java virtual machine bug was reported only for Windows.