I ran into this issue the other day. Yesterday I had to spend some time post processing a part to remove this 1st layer swell because I forgot to account for it in my model.
On the Gigabot prints I typically get a swell of 0.18mm on the perimeter of the 1st layer when I print a 1st layer at 0.27mm layer height. This also occurs on the other printers I use, but the swell is slightly different, and of course varies depending on layer thickness, leveling, etc.
All we need is a 1st Layer Horizontal Size Compensation setting that's only for the first layer. It would work exactly the same as the current Horizontal Size Compensation setting, but only be applied to the first layer.
An optional check box such as Relative Compensation could make the 1st Layer Horizontal Size Compensation relative to the current Horizontal Size Compensation setting if checked, or be completely independent when not checked. Both relative and independent are useful. If relative, the 1st Layer Horizontal Size Compensation value would simply be added to the Horizontal Size Compensation value when printing the 1st layer. If not relative, the Horizontal Size Compensation value would be ignored for the 1st layer.