Why could the A320neo carry much larger engines without changing its structure?

The A320neo carries new, more efficient engines: the Pratt & Whitney PW1100G and the CFM LEAP-1A. The A320 gave engine designers enough ground clearance for the PW1100's fan to measure 81 inches in diameter, an ideal size for the thrust class the A320 family needs. That 81-inch fan was made possible by the A320's design. Compared with the Boeing 737, whose shorter landing gear leaves Pratt & Whitney without enough ground clearance to design an engine with an optimally sized fan, the A320 posed far fewer sizing obstacles.

The A320neo retains 95% airframe commonality with the existing A320, which ensures it high dispatch reliability and minimal changes. Even so, the aircraft received additional wing and fuselage reinforcement to accommodate the larger, heavier engines.

If Boeing fitted a geared turbofan to a 737, the design would require a fan 10 to 12 inches smaller than the A320neo's. Any engine built for the current 737 would use a fan of 69 to 71 inches, compared with the A320neo's 81-inch option. The 737 MAX's LEAP-1B has a somewhat smaller diameter than the LEAP-1A, 68.4 versus 78 inches, to fit the 737's lower ground clearance.