Here's one small piece of data that I semi-accidentally collected. Let's go through part of Gygax's classic Temple of Elemental Evil module, convert the various rooms to the 3rd Edition "Encounter Level" system (which has a lot of robustness to it), and see what we get:
- Moathouse: Count 16, min 1, max 7, mean/median 4, stdev 2.
- Temple 1st Level: Count 31, min 1, max 10, mean/median 5, stdev 2.
- Temple 2nd Level: Count 34, min 2, max 11, mean/median 6, stdev 2.
Where does the discrepancy come from? Well, to begin with, in the 1st Edition days we expected a party size of about twice as many players at the table than we do now (and this is consistent in the foreword of all the modules at the time). In the EL system this doubling accounts for about a 2-point difference in the EL. (Meaning that a double-size party of 1st level could expect to see encounters of EL 3 in that edition.) But that still leaves a 2-point gap in the philosophies: depending on how you interpret that, it means that Gygax expected a "normal" encounter to suck up 40% or more of a party's resources -- and I'm guessing quite likely result in one or more party members' deaths in any encounter.
So by interpolation, we can use these numbers to broadly adjust expected character level today if we have smaller numbers of players. For example, we could send a group of about 4 3rd-level PCs into the moathouse, 4th-level into the upper works of the temple, 5th-level into the second level, etc. This assumes Gygaxian levels of difficulty, obviously; if you want to "safety bumper" the proceedings as in 3E, then you'd raise the PCs by another one or two levels. (Noting in either case that there are always some significantly higher-level encounters that you need to deal with very strategically, or simply avoid.)
P.S.:Ever notice how common it is for adventures to rather verbosely suggest adjusting monster numbers appearing in the first encounter but not anywhere else? Holmes does this in his Sample Dungeon area A; Gygax does this in the module T1 Moathouse area 1. Can you think of other examples?