While the Allies do have more income, the Axis starts with more forces on the board. The Axis must quickly leverage this force advantage into an income advantage by gaining territory in order to win.
Actually, I believe this is not true. Allies start with more production, more saved certificates (same as the production, as usual), more units value and more units number.
However, the Allies advantage in production and certificates is much more pronunced than the Allies advantage in units value and number. So, I’d agree that the Allies have a comparate advantage in production and the Axis a comparate advantage in starting forces (even though the Axis has an absolute disadvantage in both).