Actually Truman Lake is bigger than the Lake of the Ozarks. Look at any map and it's easy to see that the two lakes appear to be about the same size. It turns out that land-wise there is only a slight size difference in the surface area each lake takes up, but Truman Lake is slightly bigger. It is the amount of impounded water that is remarkably different.
Truman Lake Lake of the Ozarks
Surface Area: 55,600 acres (87 sq.mi.) 55,000 acres (86 sq.mi.)
Max water storage about 1.3 Trillion gallons about 617 billion gallons
source Ameren UE ML083360080
As we can see, despite covering roughly the same square mileage on the surface, Truman Dam holds more than twice the amount of water! This indicates that, in general, Truman Lake is twice as deep as Lake of the Ozarks.
Note: The 617 billion gallon storage rating for the Lake of the Ozarks is below the "officially" stated storage of 650 billion gallons.