How come my memory card doesn't hold the same amount of space on all cameras?

my empty memory card on my old casio exilm can hold 804 pictures. on my new pansonic lumix, it can only hold 404. why is this?

It's because the files are twice as big on the Panasonic. Moreover, different cameras will compute the available space differently, so the counters become more accurate as the card fills up. For example, my Nikon SLR may report initially 367 photos, but in actuality I sometimes have been able to fit over 400 images on the card, because the files ended up being smaller than the camera estimated.

So:
1) All of those readouts are estimates
2) the estimate will be affected by the ultimate size of the files
3) bigger files = more space used = fewer photos.
4) the counters will get more accurate as to shots remaining as the card fills up.

I would guess that your new camera is taking higher resolution pictures - meaning that the files are bigger.

It depends on how big the camera is, not the memory card.

In the same camera if setting higher or lower resolution the imformation also telling you that less or more images in the same card.