User blog comment:Xilinoc/Ch. 610 - Mausoleum of Skulls/@comment-25893842-20141219100733/@comment-5693502-20141219151432

Ichigo is a Fullbringer because his mother was attacked by a Hollow before he was born.

Ichigo is a Shinigami because his father was a Shinigami.

Ichigo is an Gemischt Quincy because his mother was a Quincy.

The manifestation of his Quincy powers just so happened to resemble Yhwach. I'd probably hedge my bets that if Quincies had inner worlds their manifestations would look exactly the same or similar at least.

The only reason I can see for why Ichigo has an extremely fast growth rate is that he is the main protagonist.