Very mobile? Kind of a contradiction, but the question stands. Ronald Wright could be very, very slick without being very mobile later in his career.
I'd think James Toney would be a good shout for this, especially as the weight began to pile on. The way he could still slip and roll and counter with his budda-like heavyweight size impressed me.
I'm thinking Whitaker, not saying he couldn't be mobile, his footwork was splendid, but he was economy of motion IMO and often outclicked people in exchanges without moving all that much. Maybe i am misinterpreting to.