One thing to remember is that while scale is based on a constant mathematical formula-it's a ratio-that doesn't mean that any two manufacturers interpret it the same way, or that a single manufacturer interprets it consistently within his own catalog.
Someone mentioned Monogram, for example, and if we look at the aircrew figures Monogram included in its kits, from the pilots in the late 50s and into the 60s, to the aircrew in the large bomber kits and others in the 70s, we can see that while they're all sold as 1/48, the earlier figures are definitely smaller. The Avenger crew is practically 1/50, for example.
And when we take scale and compare it to size, there is even more inconsistency, regardless of the math we might care to do.
The thing to bear in mind is the proportions of any two figures. The classic way to establish proportions of a normal human figure is to divide its height by the height of the head, and taking 8 heads as a normal proportion. Now, as some have mentioned, we are all of different height, and any normal group of people will show variation. But within that group, some things will be so similar as to be the same-like the size of everyone's head. Others will be similar in proportion, such as hands, feet, and limbs. It's the variation in the length of limbs that gives us different height. But a person who is 5' 6", for example, will have a head just about exactly the same size as a person who is 6'. Their trunks will also be close, and their arms and legs will show the most difference.
What I do is to take figures I want to use, and compare them to make sure they look natural, that the proportions are normal, when I decide to put them together.