# Why is methyl group more electron-donating than tert-butyl group?

As title says, why is methyl group more electron-donating than tert-butyl group? The context behind this is stabilization of conjugate base. (http://www.khanacademy.org/science/organic-chemistry/organic-structures/acid-base-review/v/stabilization-of-conjugate-base-iii look at 10:51)

Consider the tert-butyl group. In a C-H bond, the electron density is directed towards carbon. Methyl group, which has three hydrogen atoms attached to a carbon, pushes the electron density towards $\ce{C2}$ of tert-butyl acetic acid. So do the two other methyl groups. The amount of partial negative charge accumulated on $\ce{C2}$ of tert-butyl group (has 3 methyl groups) is more than that on methyl group (only has 3 hydrogens) of acetic acid.
The more negative charge on $\ce{C2}$ in tert-butyl acetic acid destabilizes the conjugate base, compared to a simple acetate ion. Hence acetic acid has less pKa than tert-butyl acetic acid.