Unwrap model with UVs at proper size? I have the model of house with seams on it.

When I'm trying to unwrap it (U > unwrap) It unwraps wrong. 

Some polygons are bigger then others, and I don't know what to do.
How do I unwrap it proportional?
this happens because either : 


have edited the mash after unwrapping
have scaled the object after unwrapping


to resolve this :


apply scale :  Ctrl+A and select scale
unwrap the mesh again