Why some mp3 files are larger?

This can depend on many factors. two of which i have listed.

1: Compression / Format. Eg. Mp3, Wav (RAW sound), WMA, AAC ( Advanced Audio Coding )

2: How long the track is.

Some files are larger because of the listed format's. formats such as Wav and AAC are known as Loss less format's meaning that there is no loss in sound quality

some others such as Mp3 WMA are known as Lossy format's because they loose sound quality.

The way these "lossy" formats work is by stripping out bits of data that the program sees as "not needed" there for having less data to save.

Not forgetting that the tracks length also means there is more data to save resulting larger in size as well.