The .aep file is just your AE project file. At its more basic, it's a database that keeps track of your assets, how you use them in a composition and the effects you layer on top of them, it's generally going to be small. The mp4 is the output. Factors like composition size 1920x1080, frame rate, bit rate and complexity of each frame all contribute to the final size of the mp4.
"I have not failed 700 times. I have succeeded in proving that those 700 ways will not work. When I have eliminated the ways that will not work, I will find the way that will work."
---THOMAS EDISON on inventing the light bulb.