I think OP is trying to account more for the fact that a larger group of adventurers reasonably will attract more attention than a smaller group. This isn't a tailoring of difficulty persay, but more of an expression of the amount of "noise" the adventurers make going through hostile territory.
In this case I would advocate rolling multiple times on the encounter table. A nonlinear buildup would probably be accurate, so let's say we roll a number of times equal to the square root of the size of the party, which is more or less the party's cross-section if you imagine them as a vaguely circular blob moving through the region.
This means that a party of 2-3 adventurers gets encounters about as often as the rules suggest. A party of 4-8 adventurers gets encounters roughly twice as often and sometimes they run into twice as many bad guys.