File size problem when encoding mov to flv

I have 5 Quicktime .mov’s which after encoding (using the Flash Video Encoder with Sorenson) to .flv’s have all doubled in size…is this normal? surely not? why? and how? I’m thinking now to stuff the player Ive built in Flash and just embed the .mov’s within the .fla, I dont want to, but the fact that they are now 10Mb, for example instead of 5Mb, worries me…any thoughts?