If you want to avoid a double stack, you'll want to avoid any G30 variation as they are slightly larger than the G26 double stack you already carry. To go thinner, you should check out the already mentioned G36 & Shield .45 or the XDS or Kahr in .45. Of course, then your capacity is really hit. Just switching from 9mm to .45 already reduces capacity. Even more so if going from double to single stack. If you're carrying the 10 rnd mag in your G26 and find it printing too much and too heavy, the 30S will be more of each (even with the smaller 9 rnd mag).
Still, you asked about the 30S. I owned a G21 years ago and never got over it's super blocky feel. Ultimately, I sold it off. After the G30S came out, I found it to be everything I wished the G21 would have been. The narrower slide, the SF frame, the option of the fairly concealable 9 round mag. All good! Fairly compact, but not small. When I pick up the G30S, it always feels "too big" for my medium sized hands. When I shoot it, it performs just fine as compared to slimmer gripped pistols.
I liked the G30S so much, I bought a G26 last year and love it, too, and it carries better. When either of those is still too bulky, I drop down in weight and size to a micro .380.
G26 & G30S (each with extended mags; although, I actually carry with the flat bases):
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Bodyguard .380
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Personally, if you already shoot 9mm and find the G26 too big at times, I'd recommend you drop down in size to a G43 rather than up in size and caliber. In a very small single stack, you won't gain much by the move to .45, but you can maximize capacity, esp. with MagGuts spring kits like I use for my BG380. A G43 or a Shield 9mm may suit you better. Just remember that the extra round in the Shield makes for a longer grip that prints more. Even the G43 is longer than the G26, but only by about a baseplate's thickness.