Causes for level size to increase?

Hi,We are developing game for mobile.

It is kind of racing game so we have set up race tracks (not circular its straight long tracks).

The level size for each scene is getting above 8mb (check via Build Report Tool), obviously it is too much as we will have lot of levels and we do not want to go for DLC thing and want to restrict actual apk and iap size to below 50 mb.

Could you guys elaborate on what may cause the scene to increase in size? All textures sizes in total after compression is only 2.51mb , so texture is not the problem.

Thanks.

try too avoid too much polygons