My experience has been Marathon Weekend is bigger (more runners/events), and thus everything (expo, etc.) "feels" bigger — feels like runners fully take over WDW for a few days! Plus it's the only runDisney race weekend with a full marathon which brings out a different crowd. The theme is straight-up "marathon" and the main Disney characters (Mickey, Minnie, etc.), plus sometimes anniversaries. Marathon Weekend is the one where I get the most "runner" vibe. It feels unique among the race weekends.
The other races seem smaller in comparison by my experience. Still excellent!! But they all put a major focus on their race theme (princess, star wars, wine/dine, etc.) and the theme typically permeates everything. Especially the expo, merch, start/finish/course features, photo ops, etc.
Princess feels like the runner up for that Marathon Weekend bigger event "vibe," though Princess still feels smaller (I assume it is smaller? don't know the numbers). And the princess theme is omnipresent. Ladies everywhere!!! (Men participate, of course, but women predominate!)