I'll guess you mean for TOS Heroic (because in 900+ gear Normal is trivial).
Typically, on Heroic when you get down to smaller group sizes, some of the mechanics are significantly harder to deal with in small groups because of how damage done by ability X, or number of pools needing to be soaked, or "insert mechanic per number of players" scales.
The benefit of a smaller group size is that you have more space to work with per person and it's less visually hectic so often easier to isolate problems without needing to check logs. This may be a benefit on Mistress (because RIP FPS on that fight in large groups) and Maiden (fewer people to fail and spread bombs throughout half of the raid.
Most of Heroic wasn't particularly hard for a NH Mythic geared group, my guild is undergeared too as we had minimal farm last tier, but we cleared 10/10NM and 9/10HC yesterday. When we got to Fallen Avatar, our 30-man starting team had dwindled down to 13 and we really struggled with a lot of very close wipes:
- In Phase 1 there are 3 pools to soak, each pool needs at least 2 people to survive, but 3 to make it more comfortable for the healers.
- With a 13 man group and the DPS comp we had, we needed to 2 heal it to stand a chance of beating the DPS check before getting wiped out in Phase 2. This felt really tightly tuned for a Heroic encounter.
- One of our DPS finished work and got back online for a final few pulls, so we could switch to 3 heal with a 14-man group, making the whole thing much more comfortable. Still tightly tuned at our ilvl though.
- In Phase 2, there are 3 sets of the soaking/knockback debuff that go out even on small raid sizes. This causes havoc later in the phase when the damage the raid is taking per second is ramping up. We got around it with the class comp we had, by saving immunities for later in the phase (Feral Druid/Hunterx2/Magex2/Ret/Hpala/Rogue all able to solo them). But this could be very challenging in a small group with a less optimal comp for solo soaking.
I am confident that when we reclear on Tuesday before reset to get the people that missed out a kill, with a 25-30 man group, it will be:
- Significantly less tight on the DPS requirement
- Significantly easier on the positional requirement to ensure everyone is in healing range (ideally of multiple healers) whilst dealing with all the other mechanics.
- A lot more hectic and probably more deaths, lower execution score - but still a kill.
I'm guessing KJ will be a similar story, but we'll see when we have a look at it tonight
The above is just an example, but it's been seen time and again in previous tiers as well. There are undoubtedly some bosses where smaller group sizes are a net benefit. However, usually on the tougher encounters and at the very least more often than not on an average encounter, a large group size makes the Heroic boss easier to plough down.