Hey there, servus!
Hard to say why she stayed small. But that's the crook with Autos. Once slowed down, b'cause of stress or what soever, your chances to catch up are small.
But anyways, she looks healthy and has a nice colour. Maybe it have been too cold for her in the first 2 weeks. Ladies tend to build bigger Leaves when it's cold.
I'm growing a Banana from Zamnesia atm. but it's photoperiodic. She produced also huge leaves the first weeks. But she had no troubles at all... as long as she looks healthy and takes up her meals I wouldn't worry that much... just keep going like you did. Good luck and have fun!
Servus✌️