If two amounts are proportional, they change at the same rate so that the relationship between them does not change.

Weight is proportional to size. 重量与大小是成比例的。

How far you go is directly proportional to how fast you go (= as one thing increases, the other increases at exactly the same rate). 走多远与走多快成正比（=一方增值，另一方亦以完全相同的比率增值）。

There's a false idea that physical prowess and intellectual ability are inversely proportional (= as one gets bigger, the other gets smaller). 有一种误解认为体力和智力成反比（=一个变大，另一个就变小）。