I am having the exact same issue with one exeption, I'm trying to convert a 10sec long mpg into a FLV. LIke you in CS4 it took just seconds to convert and the final FLV was about 1mb, if that. Now the same 10sec mpg in CS5, even after trimming as much as I can from settings, i'm still looking at 900mb, it starts at over 3000mb. I'm doing the same thing in 5 that I was in 4, web 640x480, and disabling the audio(which helps by a few hundred mb). The absolute best I can get is 70mb, and the conversion process still takes about 2.5 hrs to complete. If you hear back on this please pass it along to me, if I don't notice.
How are you attempting to "convert... into a FLV"?
Are you embedding the video into the main timeline?
or did you use the Adobe Flash Media Encoder to convert to .flv?
If you used the Media converter (which would be the correct and suggested way), what video and audio bitrate did you use?
Every video requires different encoding settings.
What are the specs of the videos you both are trying to convert? Tell us the resolution, frame rate, duration, if stereo audio is necessary and the nature of the video (high speed motion (high bitrate) or a talking head (low bitrate)).
It will be much easier to suggest settings once we know these stats.
I wrote back on the forum post I actually started and it has every step I took, with screen shots (jpg) and a link to the video (13mb or so and 11sec long)