Power Brake Master Cylinder Question?

[72K20]

Needing to replace the master cylinder and the old one has a bigger front chamber and smaller rear chamber but all the replacements from cheapo auto parts stores seem to have the chambers the same size. I'm no hydraulic engineer but it seems the front one is bigger for a reason (doing 70% of the work maybe?!?!?). Am I being to worried or do I have good reason to keep looking for the exact replacement? It also appears the brake lines are different diameters even... could this be true? Sorry in advance for all the neophyte questions.....