Texturing - one big one or many small ones?

Hello, I am developing game, and in unity I always get stuck with optimization. Games became lagy and not playable.

I wonder which is better, to use many small sized textures or put everything inside bigger ones.

Now I have street with 10 buildings, I use in total 10 textures as well. All textures are compressed to maximum and doesn't take more than 1mb in size, but game is still very lagy. Meshes are also optimized well, no necessary polygons etc.

When I read online, everyone advises to use bigger textures - to put as much as possible in one texture file. Today I opened some big games like GTA IV and noticed that they use very small textures and many. Everything is separated,in them, especially with environment mapping.

1 Can someone advice me, on which picture file type to use. I use Jpg for diffuse bump map, if I need transparency I use tiff which is way bigger file type.. ?

2 Should I use big picture files like 4096x4096 ~ 1 mb or I should go to like GTA IV uses ~256x256 ~80kb ?