HappyTail's answer is about the best, but here goes-
I'e been anal obsessed since I was a child. Even tho I started small (my hairbrush handle was my first crush), almost immediately I wanted to start going bigger. I don't understand the desire, but weeks into discovering it, I needed the next size larger. Then the next...
I bought a horrible hard plastic vibe when I was about 13. It almost turned me off anal. Not much older (you have to be 18 to get in a sex shop, but I don't think anyone is expecting a teenage girl to walk in), I discovered there are actual devices made for bums! I bought a medium size plug, about 1 1/2" in diameter. I went into a coffee shop washroom and rammed it home, with just a bit of spit. The sudden intrusion hurt like hell, but I was hooked. That was forty+ years ago.
I'm still obsessed by size, and despite some long breaks, have always tried to get a plug that's just a bit bigger into me. I can easily go all day with a pretty huge plug in, and for just lazing around the house I can quite easily manage my 4" monster, and a couple of times, 4 1/4" although that one still makes me hop around swearing.
I never try to justify it. I love having my ass stretched to new limits. I've sworn many times that I wasn't going any bigger. Didn't work.