WAV has a constant bit rate. That means even the silent parts are stored as 0 Hz and occupies space in the container.
FLAC on the other hand has a variable bit rate and eliminates the silence parts and stores only the part of the audio between 1 and 655,350 Hz. If any audio frequency in that range is not detected, say 0 Hz or anything above 655,350 Hz, FLAC treats it as silence and does not store it. As reference, humans can only hear frequencies in the range between 15 Hz and 20,000 Hz, which also narrows with age.
So if a song is long but has a lot of silent parts, then its FLAC size would be significantly smaller than its WAV file would be.
The reason why some audio stores offer both WAV and FLAC files is that audiophiles are particularly finicky about quality and they sometimes feel that FLAC, being of smaller size, is inferior to WAV.
I also feel that way, although I am perfectly aware that they deliver the exact same quality. Not getting the WAV file makes me feel that something is missing, which in fact is largely untrue if in reality, I will only be missing something that is not there and something that nobody in this world would be able to hear: even the bats.