First off, there was some selection bias in the OSR modules listed - I picked ones I've reviewed, and the reviews were chosen because I've either run them or was most interested in them. There are a lot of good ones still in the queue (and some I'll wait to review because they might be in the near future campaign).
One thing that jumped out is that OSR modules I chose tend to feature less encounters and gold, with a lower encounter density. I find this leaves more space in the module for exploration; some of those TSR modules are grind-heavy. YMMV, but I see this as a good thing. I've been running an OSR-heavy campaign and my players tell me it's been some of their favorite games - modern publishers are definitely doing something right.
Another difference is that modern adventures purport to be built for the party of five, whereas TSR often advised a party of 6-8 adventurers, plus henchmen. I'm sure that plays into why the TSR modules are treasure and encounter dense. There's also a difference between "tournament modules" and "campaign modules".
Part of why I built the list is because our goals in doing Gothic Greyhawk as a campaign was to get to the point where I could run Against the Giants; since I'm seeding modules in a sandbox setting, having a handle on the "weight" of the modules will help me gauge the pace of advancement. I'll be adding more in the coming week - Ravenloft, Night's Dark Terror, the C-series, and then whatever I review this week.
Check out this factoid - a party of 9th level characters will have accumulated like 2.5 million to 3 million gold pieces along the way. I know smarter folks than me have bemoaned the D&D economy - should be fun figuring that one out!
Something that became apparent while working through S4 Lost Caverns, or the various G-series modules - those weighty treasure volumes will require massive logistics to actually get the treasure out of the mountains - I'm seeing a small army of mercenaries, porters, mules, and wagons will have to go along just to cart off the spoils.
Something to look at next - are there significant differences in monster XP between editions? Pat in the previous comments pointed out that monster XP is less than 10% in BX, where I thought AD&D was weighted heavier. Also, does Basic BX or the clones address the lack of magic item XP by upping the amount of treasure?
I know that many of us convert between editions on the fly when running these adventures in our rules of choice, but there might be some subtle tweaks to experience that bear further investigation.