Different resolutions = Same Filesize? (Pictures)

[Dot50Cal]

I ran into a rather odd discovery today. While encoding to my standard 400x296 resolution I decided to see how much larger a 480x380 resolution file would be. I was shocked when they were nearly identical in size. Heres some screencaps

480x380:
http://img91.exs.cx/img91/7766/480x3805fa.jpg

400x296:
http://img91.exs.cx/img91/7154/400x2969wb.jpg

Now, I blew them both up in Vdub to 800x600 to create a comparison to see the differences. Naturally the 480x380 one looks sharper but why isnt their a filesize penalty? I can only assume im losing quality here but where? I have both set to the same bitrates. Heres my capture info:

Encoding from Huffy AVI using Nandub and Divx3 (Please dont comment on how obsolete this is, Im set in my ways).

800x600 blowsups:

480x380 - 800x600:
http://img91.exs.cx/img91/9875/480x380800x6001ft.jpg

400x296 - 800x600:
http://img91.exs.cx/img91/6377/400x296800x6008fq.jpg

[gircobain]

If you set both to the same bitrate, how could they end up with anything but nearly identical file size...?

Seems quite logical and straightforward to me... :devil:

[Dot50Cal]

Yes but shouldnt they be affected by the extra pixels? Surely a 320x240 file @ 900 bitrate will be smaller than a 640x480 @ 900 bitrate?

Edit: ok its been a long day...So basically im losing quality on the higher resolution one, correct? Yet when compared the larger resolution one looks better..Hows that work?

EDIT2: Finally something makes sense! I did this to another part of the file and the files are different in size!
400x296 - 2.97MB
480x380 - 3.80MB

So whats up with the other one??

[gircobain]

Um...no?!
Bitrate by definition means a definite amount of bits spent every second
Since both encodes allegedly (sp?) have the same length in seconds, they should turn out the same size regardless of resolution
Now which encode has better quality is subjective and arguable

Edit: In response to your edit, I'd like to ask you to check for the actual bitrate and time length of both clips
File size has only to do with bitrate and time length
Resolution has to do with quality, but nothing with size

[Dot50Cal]

Can you explain the Edit 2 then? Why do those (much shorter video) have filesize differences?

[gircobain]

You may find out that on the smaller clip, the codec did its best job and can't compress it any further, so the actual bitrate is smaller than that you asked for
Either that or the clips have different time length...

[Dot50Cal]

Well I suppose a de-interlace filter could also add to the size right? I have it going before the resize so that could be it. Both clips were from the same clip though so its the same length.

[gircobain]

You see, average bitrate is a concept similar to average speed
If you drive at a certain average speed, for a certain time, it doesn't matter which route you drive through, nor how big is your car, in the end you have ridden the same distance
If two given clips have the same time length and different file sizes, they necessarily have different average bitrates
Notice that I'm talking here about total bitrate - so you should account for audio bitrate and container overhead as well

[Dot50Cal]

Yeah I got it, Boy looking back on that I feel stupid :(