Two sets of
numbers are proportional to one another when one set is a constant
times the other.

For example, {1, 2, 5, 7} and {3, 6, 15, 21} are in proportion,
because the numbers in the second set are 3 times the numbers in
the first.

Scale models are made so that all the dimensions of the
model are the same constant fraction of the real object, so the
dimensions of the model are proportional to the dimensions of the
real object.

In mathematics two variable quantities x and y are
proportional if, for all values of x, y = kx, where k is a constant.
This means that if x increases, y increases in a linear fashion.