when i try to convert to WAV the 2.28 MB becomes 10.1 MB
wav is an uncompressed format, the file is always going to be bigger.
BGM's aren't hard to make, just have to know what you're working with.
The way I do them it's a bit different to this post but fair enough as its an old thread.
You'll need Looping Audio Converter 1.3, Usagi 3DS theme editor, some kind of audio editing software or DAW (I use FL Studio)
and ctrwave converter.
Get the song you want as a bgm and pop it onto the audio editor, make it as long as you want without going past 2 minutes, more than that will make it so you need smaller sample rates to convert lowering the quality (1 to 1:30 minutes should be the best range for quality)
once you have it edited export it as a wav and save it in the ctrwave folder for later.
Now follow the instructions for ctrwave converter: open a cmd window inside the ctrwave folder (type cmd in the file explorer or right click and open it). when done drag the ctrwave.exe into the cmd and input this with the filename you have for your BGM:
--pcm16 inputwavfilename.wav -o outputfilename.bcwav
that should convert your wav to bcwav, and being in this format lets you change the sample rate in Looping Audio Converter later. After this just get the bcwav version of your BGM, add it into Looping Audio Converter, check the option to put all channels into 1 file and change the sample rate to a lower one if the output file is still bigger than 3.3MB (the limit is actually 3.37 something so just make sure its below 3.29 and you'll be good.
Check the use Brawllib decoder option, the Output format should be BCSTM (ADPCM) and that should be it, as someone said already, mono only comes out one side so make sure to export it in stereo, even if it takes more of a hassle to shrink the size as mono only comes out one side instead of merging channels.
This might be a bit more convoluted than other guides but it's the best way to go about it if you want your BGMs to come out without issues.
Hope this helps!
(sul on themeplaza)