That depends entirely on the ratio between how much you already had invested at the start of that time period versus how much you invested throughout that time

That depends entirely on the ratio between how much you already had invested at the start of that time period versus how much you invested throughout that time period. If the ratio is nearly infinite (i.e., you had a lot to begin with and hardly added any more) then yeah your S&P500 balance should've grown about 100%. But if the ratio is nearly zero (i.e. you didn't have much at all invested at the start but contributed heavily since then) and for simple math we say that the S&P500 grew linearly, then you should expect only a 50% growth because only the early contributions rode that train while the recent contributions hardly had a chance. In reality it'll be somewhere in between. And as pointed out by others, this assumes S&P500. If you're in a lifecycle fund instead, that's a bit like having mostly S&P500 when you're young and hardly any when you're old.