They are small, about 32x32x32 quants size. They actually have high-res textures, with a size of 512x512. When I removed the texture completely from one of my cubes, it used a much smaller amount of memory, but removing the texture is not a solution
I tried one of my cubes at 16x16 texture size, but it didn't help much.
The problem is, the big amount of cubes I need in my level. If I build 3 houses, the game starts to lag, and uses about 600 MB of RAM. It's about a few hundred cubes, maybe 200-300 of them.