What qualifies as 'substantial?'

[MikeyIs4Dcats]

I was told today I'm in line for nearly a 20% raise, assuming all the approvals go through. I'm keeping my fingers crossed, as the extra 5 figures will come in very handy with a baby on the way.

BTW, why does substantial have the root prefix sub- which typicall means lesser or below but means a lot or increased?