MP3 is a compressed format, and it is not possible to do sophisticated editing or processing without first decoding (de-compressing) the file.
Audacity works internally with "32-bit float" data, so to minimise losses, it decodes imported files to 32-bit float data. Editing and processing can then be done without loss of sound quality.
MP3 files are typically around 8 times smaller than the uncompressed 16-bit data.
32-bit float is 2 times bigger than uncompressed 16-bit data.
It is therefore expected that when an MP3 file is decoded, the decoded data is likely to be around 16 times bigger than the MP3.
During working in an Audacity project, Audacity maintains a copy of all data that is changed, so that while the project is open you can undo what you have done in the project. So for example, if you have a project that contains 10 MB of data and you apply an effect to all of the audio in the project, then the space used by the project will increase from 10 MB to 20 MB. The original 10 MB is retained so that you can undo the effect. Apply another effect, and the project will increase in size by another 10 MB. On closing a project, the Undo History is deleted to save space.
Working with audio (or video) media requires a lot of working space for data.
If you only want to do very basic editing, such as trimming the ends or adding a fade-in, you may be better to use a dedicated MP3 editor such as mp3DirectCut, which can perform simple edits to MP3 files without decoding them.