The Glock 30S is a great gun with a slightly more narrow slide than the standard Glock 30. However, the grip is the same width and this is what sits above the belt. I do recommend the Glock 30 because it's a great gun.
The most compact pistols on the market that hold eight or more rounds are the S&W M&P 45 Compact (8+1, with 10+1 option) and the Springfield Armory XD 45 Compact (10+1). The SIG P220R Compact holds 6+1 rounds and the Springfield Armory XDs holds 5+1. The Glock 36 holds 6+1 rounds.
The M&P 45 Compact, XD 45 Compact and SIG P220R Compact are all approximately Glock 19 size--which makes them quite concealable on the belt. The Glock 36 is also approximately the same size.
The M&P and XD 45 are more controllable than the small Glock 36, which I found to be very snappy. I have not fired the SIG P220R Compact, but I imagine it will roll under recoil like the standard P220 does.
I really tried to like the HK USP 45 Compact. The trigger was not all that great in double action mode. There are nine variants, including two that permit "cocked and locked" carry. The frontstrap checkering made the recoil feel worse than it actually was. It is a very wide gun and the magazine baseplate does not need the finger rest. Perhaps the HK 45 Compact may be better; take a look at it.
Of all the choices I listed, I'd choose the S&W M&P 45 Compact (with APEX trigger parts and Trijicon night sights). I'd carry it with an eight round magazine inserted with two spare 10 round magazines on my belt.