The AutoScaleMode tries to re-size the form according to the system DPI. For laptops, Surfaces, etc, which have very small, high resolution screens, the DPI is often set to 125% or 150%. The AutoScaleMode property then tries to increase the size of the form to match the increase in the size of the text. I don't remember the intricacies of which option does what. You'll have to try them out by setting the different options and changing the DPI on your computer to test it.
I do know that at 150%, the text on WinForms looks like trash. Instead of rendering the text at 1.5x the size, it actually renders the text at 1x to a bitmap and then scales the bitmap up to 1.5x. This makes the text look blurry and blocky, just like scaling up any bitmap.