Does it share the DPP footprint?

How much smaller, bulk wise, is the Shield vs. the DPP?

The Shield and DPP share the same mounting pattern but as far as I'm aware they are not dimensionally identical. If you send off a pistol and specific optic to be milled it won't necessary fit the other. Pistols with factory optics cuts are more tolerant as it's more about the mounting plate you use and you're going to have some space in front and back compared to milling a slide to a specific optic.

Size wise my RMSc provides a significantly larger viewing area than either Trijicon RM07 or Holosun 507. Height wise the RMSc is short enough to work with standard height iron sights while the other two require suppressor height irons.

The DPP and Sig Romeo 1 share the same pattern. The RMSc matches the Romeo Zero.