Yeah it's just one of those things I try not to think too much about. He's insanely strong, has massive leg muscles, and incredible kicking speed.
Ichi's actually one of my least favorite characters in the manga. All the crying, erections and masturbating is too much. I did like the look into how a Yakuza clan is run, and how the Anjougumi is full of so many interesting characters. Kakihara, Kaneko, the twins, etc. Ichi running through the entire gang felt like something out of Max Payne; one man's vendetta against a criminal enterprise, only spoiled by the S&M-centric showdown between Ichi and Kakihara.
I would have loved to see a follow-up manga that explores Jiji's past, and why he sent Ichi to take out the clan. Also maybe seeing Kakihara before and after he got his mouth scars, etc.