SiG has the two different sized grip pieces. Other than that and adding something like the Hogue Universal grip sleves there aren't many options. That said, the previously mentioned only really help those with larger hands.
I've read on other forums some that have done things to Glocks to reduce the girth. However, I don't think I would do so, personally. I don't know what engineering data went into the design of them, and other polymer frames, that required them to be the size they come in. To reduce their size, not to mention doing so with a woodburner tip by re-stippling them may reduce the integrity of the design. I am not a polymer engineer and I didn't sleep at a Holiday Inn Express last night. Just seems that doing so without a LOT more information and knowledge is far from prudent.
The SiG offers a little light at the end of the tunnel. The fact that the grips are a seperate component from the frame rather than being integral as a one piece mold means reduction is an option. However, whether it's done by re-stippling, sanding or whatever would be one of the options you need to seek more information on.