flv is not a codec - neither are avi, mkv etc etc
they are containers
and can be encoded with various codecs - you can take a movie, compress it using a divx codec (for example) and then turn it into an avi or mkv or whatever you like.
flv's can be produced using a variety of codecs - including sorenson spark, on2 vp6, screen video, screen video 2 or h264, depending on the software being used. each of them will yield different results, with differing file sizes.
you can even chose different compression settings within the same codec, if you wished - so you can take the same source, and produce two flv's from it, both using h264 codec, but with different compression settings.
it's a bit like jpg images - they can have variable compression applied to them. for example, here the big picture is actually a smaller file size as it has been more heavily compressed.
100x100 pixels, file size 6.4 kB
150x150 pixels, file size 2.8 kB