The bestiary says that when you have parties larger than 4, you must increase the XP total of the enemies to compensate. There is a handy chart and everything, so it's pretty easy. But this also comes with a warning: "Add mooks instead of using bigger enemies or it can end badly."
From my experience, TTRPG party sizes are generally between 4 and 6 PCs. And all sizes in that range are decently represented. Let's talk about a hypothetical party of 6.
A lot of monsters in any RPG bestiary will be "solitary" encounters where you can't really add more creatures without screwing up the ecology or adding another member of the same species (which would make the encounter too hard). Good examples of this are Dragons, Manticores and Bullettes, to name a few iconic ones.
When a 6-man party is at the level where they could face one of these, the difficulty is going to be a lot lower compared to a 4-man party, so the GM should need to make some ajdustments. Since adding more enemies is out of the question, you need to power it up somehow or replace with a bigger creature. The Bestiary says this is dangerous, but it can be done in some instances.
The issue is that when you can do this is pretty inconsistent. Going +1 Level ont he bestiary may result in +1s across the board, which the larger party can handle, but at other times you also get a lot of +2 and +3 increases, the latter which can end in a quick TPK. The advanced template thing they got is also a pretty big boost to monster power.
So one issue here is that monster scaling is not linear or on a smooth curve, but instead has big spikes up in power at particular levels while it's linear at others. This means that if the 6-man party is at the wrong level, they could be horribly killed by upping enemy level by 1, whereas it works perfectly fine sometimes.
I think either the power curve should be smoothed a bit, or the GMs given more guidance on handling the scaling of solitary encounters.
The other issue that compunds it is the "tight math paradigm" people been talking about a lot lately here. That +3 wouldn't necessarily spell death in other RPGs, but in the PF2E playtest it is practically insurmountable because of the degrees of success system and few ways to gain boosts besides leveling up.
If the mechanics remain as it is, Adventures should be careful in the guidance they offer GMs for scaling encounters to challenge larger/smaller groups (PFS ones often do), or the XP budget system adjustments may need to become more nuanced for these edge cases.
As it is, straying too much from the 4-man party means your only real choice is to stick to groups of small enemies and adjust their numbers accordingly. Using big monsters just results in a mess.