FLAC files use lossless compression, which means all the original data is preserved (which is why people like FLAC), which makes FLAC files smaller than an uncompressed .WAV audio file, but still larger than an MP3 file, which uses lossy compression. Lossy compression does involve the loss of some data. This is what allows MP3 files to be smaller than FLAC. With the existence of other more modern lossy audio file formats/codecs like .AAC, and .opus, MP3s should no longer be used unless it is required for compatibility reasons. The modern lossy formats offer higher quality audio files at lower file sizes than MP3.