File Size in Movie Maker

Hi

I am doing videos on YouTube and for cutting i use the Movie maker because often i don't need to do much stuff, only put one or 3 files together.

Now my Problem

Every time i save the movie the file size gets ridiculously big for no apparent reason

as a example :

I  have a recording in 130 MB in MP4 format,

all i want to do is put my intro in front of it which is 20 MB big also in MP4

When i am going to safe movie the size of the file will be over 1 GB without gaining any quality of course.

Can anyone give me a tip on how to prevent this ?