I don't understand...

So when you inflate your tyre with a co2 cartridge does that air leak back out gradually?! Sarah said it''s something to do with the size of the molecules. I had to use one on Saturday. When I inflated the tyre it was solid. However by the time I got the bike home, you could feel the pressure had reduced from when it was first inflated. :-o I don''t understand what the difference is between using a pump and one of those and why it would be any different?!