A319/A320 MTOW differences
 

Maybe that is a stupid question, but I have been unable to find a reasonable answer...

I understand that there can be different structural configurations, or interior/fuel tank configurations, for a given type of aircraft (for a 320, or a 319...). I think that is what Airbus calls weight variants.

But lets consider the maximum MTOW of each, for instance 77T for a 320 and 68T for a 319: why is it that by cutting a few frames out of a 320 (to make a 319) the maximum MTOW decreases?

Is it linked to specific structural differences, or just due to commercial differences?

A319/A320 MTOW differences
 

27,000 lbs thrust on the average A320 engine and 22,000 lbs thrust on the average A319 probably is a big factor

My 319 has a MTOW of 75,500 kg.

The huge difference is that the aircraft to which you refer would have a reduced MTOW due to operator choice. There are a number of benefits for doing so for some operators.

Commercial - I believe airway charges are a big player. Our A319s have now stbilised, but at one point the MTOW / MLW numbers were changing seemingly every month ;)