On Windows gVim:

On Ubuntu gVim (same on Vim):

Why is the padding across the two operating systems so different? Is this a changeable feature?
One difference between Windows and other systems is the default for the 'linespace' setting, which inserts one pixel line between characters on Windows. See :help 'linespace'.
Besides the answer given by Ingo, note that starting from Patch 7.4.393 Vim on Windows let's you tweak the font rendering by setting the 'renderoptions'