Compression and Limiting are very different.
We're not talking about the classical definition of limiting being a compressor with a ratio of greater than 8:1 but about brickwall look ahead limiters. These limiters "look-ahead" at what's going to come, and react in advance ensuring that nothing goes above the threshhold. Thus the name brickwall, as it's literally a ceiling that can't be passed. Because of this it allows one to remove peak transients (and more) from a digital recording, pushing the overall level higher without clipping.
When we talk about compression in mastering, we're talking about regular old run of the mill compressors as you're used to. These can be helpful in setting a tone, and gluing a piece together, but in general the level gained in mastering mostly comes from the limiting. Again, generally, of course compression, saturation, EQ, etc. count as well, but the largest part is coming from the brickwall limiter.
It's hard to say there's a rule of thumb, but normally compression in mastering is only a dB maybe 2, usually pretty slow attacks, compared to mixing, anything more is usually considered pretty extreme, and this usually leaves most original transients roughly intact, whereas limiting very clearly lowers/limits transients and may gain you 6dB in some cases or even more.
To add to the confusion, there's of course other possibilities, many people use analog limiters as well, which although aren't brickwalls, may help get a dB or two extra, in a different way than a brickwall, and there's of course multi-band compression/de-essing/acceleration limiters which all can be handy in certain situations.