15G spools fast because it has a tiny hotside. So you get boost sooner, earlier, more often, less lag. Which is better for a general daily driver. The main issue with a tiny hotside is that it limits peak flow. At high rpm WOT, the engine can only force so much air out past the hotside, even with the wastegate open. So less peak HP (usually found at high RPM's) but more usable, easily accessible power down lower in the rev range.
A big turbo won't choke the air flow on the top end, but then the engine can't spool it very easily You get more lag, you have to wait longer as the revs climb, and even then, off throttle to on throttle there's more time. You do get a higher peak HP, but it's not all win-win, you lose low rpm power and throttle response.
Some big turbos can do things to help spool earlier. Divided scroll helps, ball bearings help, exotic lightweight materials in the spinny parts of the turbo can help. As well as having a cam suited for the RPM ranges the turbo will work best at. A high revving cam isn't a great match for a small insta-spool turbo, nor is a T (for Tractor) cam a great match for a big turbo aiming for high RPM dyno queen peak HP glory.