File size when exporting?

I have a 60 seconds clip I want to export so I can upload it to Vimeo.

It's 1920x1080, but I want 1920-817. I use Compressor.
When exporting the 1080 with H.264 I get 92mb, but when when using Compressor settings I get 504mb.
I'm compressing audio to .AAC and 128 bitrate. Video is set to maximum 5500bit rate(same as the 92mb file). Still it turns out 5 times bigger file.

Why?

First time edit:

I was just playing around with the settings. And for fun I tried to change only the frame size. So I changed it from 1920x817 to 1920x1080. Suddently the size of the file went from 500mb to 78mb. Why?
And when trying to use 817 settings, FCPX can't estimate size, but with 1080 settings it can. Why?

I'm totally confused!

Second time edit:

Back at first edit, in Compressor I had to change cropping to custom in order to set the frame size to 1920x1080.
And that's when I got the 145mb estimated size.
I went back to Compressor, changing ONLY frame size to 1920x817. Compressor changed cropping to Panavision 2.35.1.
NOW I suddenly have 145mb using 817. Using the exact same settings as when I got the 504mb.

Anyone please explain. It's driving me nuts!!