Question: We have a Pathfinder group that consists of 11 people, and has been like that for a few years now. In the beginning we had a great time with such a large group and didn't mind the number, but something in the dynamics has shifted over the last campaign.

Four of them almost never show up (which we can excuse because they never show up due to work/school), but when they do they barely pay attention and end up leaving early anyways, which disrupts the flow of the game. Two of them just... don't seem to be into it anymore, and their non-enthusiasm tends to ruin the mood of everyone else. Plus a *ton* of personal problems most of the party has with one of them in particular that getting into in detail would take far too long.

The DM wants to reduce the group down to those that show up regularly and actually want to participate in the storyline, which is about five people; the average D&D party size. One of the people who doesn't show up due to work is alright with this due to his becoming disenchanted with roleplaying in general, but we're not sure about the others in the group.

So what I'm trying to figure out is how to politely tell the rest of them that we want to reduce our group? Is it even possible to be polite in this kind of situation?

Answer: 
I run short campaigns (~2 months), and when I start a new campaign I send a new round of invites. Anyone I didn't enjoy playing with in the previous campaign, just doesn't get invited to the new one. Nobody has ever given me any grief about this -- I think it's mutually understood that when I run a game I can invite who I want. :)

Your question uses the phrase "...over the last campaign", so it sounds like you have a similar structure. Can you end your campaign and then start a new one with just your core group invited? That seems like a nice low-drama approach where you don't have to specifically uninvite people.

("Have a frank and open conversation with your less-frequent players about the problems you're seeing and the best way to solve them" might still be the best approach, but I don't always feel comfortable doing that in my group, so I thought I would share what works for me.)
I used to have eight players in my campaign and I found that as the PCs got higher level it became harder and harder to keep the flow going. The multiple attacks per round and more complicated actions caused the rounds to become very long. Two have dropped out and I am now down to my original six and I intend on it staying at six. The two dropped PCs are now NPCs in the world and the party runs into them from time to time. Now of the remaining six, if someone can't show, I either run their PC as a NPC or come up with something in the story line that keeps them out of the encounters. I typically don't like players playing two PCs, I like them to focus on role playing one PC. I also have two alternate players who will take on the PC of the player that didn't show up. The rest of the players and I do a good job making sure the alternates stay in character. The absent players don't mind having someone else play their PC and I usually write a recap of the session so they absent players know what happened. Do you have alternate players to take over the PCs? Can you come up with a story line to reduce the number of players?