So just by looking at the m4 sherman, and m6 heavy tank, they both are very large compared to the tiger and pz4/t34. Why did they choose overall to make them a bigger tank ?
The three medium tanks you mention, the Panzer IV, the T-34, and the Sherman, are all about the same size:
Length Width Height
PzIV 5.92m 2.83m 2.68m
T-34 5.92m 3.00m 2.40m
M4 5.84m 2.62m 2.74m
As you can see, the Sherman (other than the longer M4A4 and M4A6 variants) is slightly shorter and narrower than the other two. The M4 is noticeably taller than the others (and the PzIV's height includes the cupola). This is due the the requirement to fit the Wright R-975 Whirlwind radial engine. The radial engine was the only engine available at the time of development that provided enough horsepower to meet the design requirements. The engine sits upright, and the driveshaft comes off the center and heads forward and downward to the transmission. So the top of the engine deck is high to clear the engine, and the turret is even higher to clear the driveshaft.
The M6 Heavy Tank is similar width to the Tiger. It is longer and taller to accommodate the Wright Cyclone engine, which was larger than the Whirlwind.