Why removing noise increases my audio file size? I took some mp3 files of people talk with noise in the background and then removed the background noise using online voice seperator tools (such as https://vocalremover.org/). I was surprised to find out that the size of my files increased (about 5 times bigger).
Why did it happen? Is there another audio format or background noise removing method that would reduce my file size when removing the background noise?
This is has nothing to do with the noise removal process but with your encoding. In order to do any type of processing with an encoded audio file you need to

Decode the input file to PCM
Process PCM data
Encode the PCM into the desired output format

Please note that steps and 1 and 3 are completely independent. The decode is determined by the input file format and the encode by the desired output file format. If you want those to be the same, you need to capture all encoding parameters from the input file and pass them to the encoding process.
In your case, you are probably using a different bitrate for input and output.