Ariel tends to be a long line because as a mermaid, you can really only find her in her Grotto, for Staging purposes

She's not like Belle and Snow White and can just walk out to her meet and greet space and then leave when her time is up. She is hidden in her "grotto" so the people in line can't see her when she gets on/off her rock...
Tink is found in person with her fairy friends (2 of them) but only in Toontown Fair. they have created a set up where you "shrink" down to fairy size as you move down the hall to see her. THen when you get in the character room, all the ojects are larger than life, like the teacup that is as tall as an adult, a stack of coins as high as the room, etc. Disney needs to make the character greetings as "real" as they can!
Oh, and I think Tink is a longer line because they play a game with the kids when they enter the fairy room, and dance around the room and sing. So its not just a "go up to Tink, say hi, pose for a pic and get an autograph" there is more to that character greeting than most! They also only let a few kids into the room at a time, so the line moves slower!