How are arcade machine boards turned into Mame roms?

Anyone know of a good read or video on how this is done? Recently I've gotten interested in Mame and how the Mame roms are created from the actual arcade machine eproms on the board. Of particular interest Is why some Mame roms are 1/10th the size of the original machine games. Here's a link where SNK quote the size of several of their games as over 100mb in size and their machine being able to handle up to 300mb sized games In 1992. Watch at 4:30 https://www.youtube.com/watch?v=K1yyAA1wgDE Then you look at the Mame rom for the game and it unpacked to a size of 16mb! So I'm curious what goes into reducing the size and why it's done by the people who create the roms. What do they sacrifice In the original game iproms? Sound bitrates etc?

Thanks In advance.

http://wiki.mamedev.org/index.php/Dumping_roms

I think there is some confusion SNK done a few multi game systems that could indeed store that amount of data but stored a few games (there was 4 cart and 6 cart varients) not just a single game

Mame roms are raw dumps, no compression done asides from being stored in a zip file

100Mb is apx 12MB, that's Megabit vs MegaByte

Mame only uses compression for other media such as CD HDD ect... in the form of chd

EDIT

Do note that mame uses a Master rom and Clone/Updates and alt regional roms the later tend to be much small as they share chips from the master rom and only contain the roms that are altered for the other releases, meaning that the master rom is the only full dump the opthers use roms from the Master rom as well as the extra alternative chip dumps stored in there own zip file