Swapping out BMPs for PNGs

It's my understanding, in general, that the PNG file format is smaller and
more appropriate for graphics being displayed on a monitor screen. Much
smaller. So I tried an experiment, in an effort to reduce the overall size
of my Captivate file.

1. I exported 5 of the background images in a Captivate (3) movie and
converted them to PNG files using Adobe Fireworks.
2. I imported the new PNG files back into the same Captivate movie
3. I swapped the backgrounds for those 5 slides, replacing the BMP files
with the new PNG files
4. I deleted the BMP background files from the library.
5. I saved the Captivate file

The file size of each individual BMP background image had been about 2500
KB, replaced by a PNG background image that is about 80 KB. I know you're
thinking that this should have made a dramatic difference in the file size
of the Captivate movie. That's what I thought too. But I was wrong!!

The result ...the Captivate file with PNG files was larger! Not a lot ...
but it was larger.

Can anyone figure out why this is the case? There are no settings that I can
find that address the use of PNG files at all.

I'd really like to get the file size of my production down, without
sacrificing quality. I thought for sure that this would do it.

Marge