Speedtest.net tests your download/upload speeds using tiny packets and bouncing them off of the nearest 'server' and seeing how quickly it can get them to and from there.
When you're actually downloading information/data/films/games from a live web server or filehost (Steam for example) there are a lot more factors. Additional factors include things like Host server load (how many others are also trying to download at the same time?), latency, line speed, data quality, etc.
EDIT: Also, internet download SPEEDS are measured in MBPS (MegaBITS per second) or KBPS (KiloBITS per second).
The SIZE of a file is measures in megaBYTES and KiloBYTES. some ISPs also but a data limit on there internet connection, also measures in various denominations of Byte.