あー見えて12さい、小っちゃいけど大きい可愛い

Being 12 despite looking like that, being tiny yet big, she's cute.
"Whatakyuti" is originally "[i]kyawatan[/i]", a slang term popular among middle-school girls, combining a slightly over-emphasized "[i]kawaii[/i]" (cute) with the childish/cutesy honorific "[i]-tan[/i]" suffix.