Most retro gamers rely on "Fair Use" for abandonware. However, companies like Nintendo, Capcom, and Sega aggressively protect their IP. If you own the original arcade board, you are legally entitled to dump your own ROMs (a process called "backup") and use them with MAME 0.144.
Later versions of MAME introduced heavy reliance on CHD files (Compressed Hunks of Data) for games like Killer Instinct and Cruis'n USA . A single CHD can be 2GB. MAME 0.144 uses CHDs rarely, meaning your ROM set stays small (around 30GB for a full set vs. 500GB for a modern set). mame 0.144 roms
Under the Digital Millennium Copyright Act (DMCA) and international law, downloading a copyrighted ROM for a game you do not own is illegal. MAME itself is legal; the distribution of proprietary code (the game's program data) is not. Most retro gamers rely on "Fair Use" for abandonware