> I always hated how the "Login" button is smaller on every website than the "Sign Up" button.

I always assumed that was because it's the least common flow. Once you've signed up, you're logged in, so the only time you should ever need the "login" button is on a new computer, or after logging out to switch accounts, or if the service offers a "remember me" checkbox (don't do that) and doesn't check it by default (definitely don't do that), or if the user genuinely wants to not be remembered (e.g. they used private browsing).

If you're having to use "login" on any regular basis, something in the overall flow needs improving.