Each arch is different in span. There are several explanations for the differing spans of the arches. As it's mediaeval it was originally built with wood and the available lengths of timber could account for the differences.
It was subsequently rebuilt with stone and the belief was that money was supplied by the various Guilds and prosperous families and what they could afford influenced the span of "their" arch.
Another idea is that the piers were sited on randomly situated large stones on the river bed. It's been around since the 13th century so doubtless could tell some tales!