According to the schematics in "Southern Steam Power", the 4800 series SRR Ms-4 Heavy Mikado engine is 2'-2" longer than the 4500 series SRR Ms-4501 when measured from pilot axle center to trailing truck center. Overall [engine +tender], the Heavy is 82'-0-3/4" and the Ms Light 77'-0-7/8" measured pilot to rear coupler.
The Heavy's boiler has 32% greater heating surface than the Light Mikado.