I'm a big fat butt.
Wow! This is almost better than i'm a big fat butt was!
The latter.
Personally, I believe that if the OP wanted only one line, he would have said "Title, and only line" rather than "Title, and the first and final line."
two big fat butts?
Let's make it:
i'm a big fat cock
Penis. R(L[M)m
i OցOj i'm a big fat butt
i OցOj i'm a big fat cock
i OցOj I'm a big fat butt.
i OցOj I'm a fig bat futt.
i'm a big fat butt is the DQN equivalent to baby got back
q , O߁r DQN deju vu!