IMO it isn't. AD&D was a much more
fiddly beast, but the design decision to make everything into a subsystem meant that it really wasn't that complex. Just full of fiddly detail.
You get complexity when you get emergent interactions and when you get choices. The stacking of modifiers in 3.X and the choices that lead to lead to a lot of complexity in 3.X - and the way that you could set things up in 4e mixing forced movement, zones, and terrain were even more complexity in combat.
But AD&D? Just go through the steps. It's not as if you have many mechanical options at all - and it's not as if the subsystems get to interact because they are different subsystems. Adding things like helmet rules, a parasitic infections table, or a wandering harlot table doesn't add complexity - it just adds stuff.
That's because 1e used tiny type with little space given over to art while 5e is extremely spaciously laid out.
If you're going by
word count then based on
this thread the 1e
DMG is over 240,000 words, making it pretty massive; the 5e DMG is just under 200,000 words. The PHB by contrast (in other words the parts the players have direct access to) was only 132,000 words - which compares to about 210,000 for the 5e PHB. Of course I'd argue that about 50,000 words that were in the 1e DMG (including things like the attack matrices, saving throws, and the enlarged spell explanations) should have been in the PHB which means that the 1e rules are very slightly smaller when combined.
Then there's the monster manuals; 5e's uses much more and much bigger art. There's a difference in size of course - but it's about 98,000 words vs
apparently 105,000.
Or in short if we go by word count 5e is about 10% bigger.
IME this is putting things backwards. People who are not familiar with the bulk of spells don't DM. This has two consequences that are actively harmful to D&D
- It is bad for the community because only people who can memorise spells like that DM, preventing them bringing their talents
- It is bad for worldbuilding because it restricts the spells that are used to only the common ones.
And "doesn't slow things down very much" still means "arbitrarily slows things down".
And here I think that you are objectively wrong and that in the long run spells are actually really hard to balance while subclasses take more work up front but are ultimately quite a bit easier to balance especially for the impact they have. This is because of combinations; a character can only have one subclass so you only have to balance that. But a character can have two dozen spells - and you don't have to just balance for that one spell, you need to balance for combinations. Spells can combine with spells while subclasses (other than the first ability) almost never combine with each other. Interactions are where complexity happens.
Or, to put things another way,
Shield is a really overpowered spell not because wizards and sorcerers can cast it but because paladins can poach it and combine it with plate armour and shields. No one can poach a level 7 class feature from the subclass for another class.
For me the bloat began when, unlike almost any other game I play, the decision was made to separate the rules into three large hardbacks rather than one.
And then there are people like me. I can do without extruded fantasy product - but I definitely want more books like Tasha's that push at the boundaries of D&D 5e.