IF you want smaller filesize, convert it to 96kbps HE-AAC, but yeah AAC is lossy format too, it will strip the detail from your MP3 again, you will get smaller filesize but worst quality

FLAC is lossless compression format, its unlike ZIP RAR and 7Z(can compress any sh*t), FLAC only can compress uncompressed PCM(commonly known as WAV) audio.
the audio converter will need to convert to WAV and then compress to FLAC, thats why your music file ended up LARGER than before