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I believe the idea is to cut through the mud and hit something a little less soggy underneath. Chunkier tyres would get clogged pretty quickly. MX bikes come to mind.
Testing true effectiveness of 33 vs 40mm tyres is a whole other thing. Saying all that, it's all swings and round abouts.
Few years ago we were all rolling on 23c tyres and 650b was an anoraks' plaything. -
Yes, they are.
There are two ways I can see to go about riding through loose ground, floating on top of it (fat bikes on snow/sand) or cutting through it (if we are looking for another sport comparison, I'd bring up the narrower tyres you see on rally cars in the snow).
The mud shedding is a function of many things, mud water content and soil type, mud bike clearance and mud specific thread on the tyre.
In my experience, a 33c CX tyre (33mm is the official UCI limit) is much more efficient than a 35/38/40 knobbly tyre in the English mud we have between Nov and April. I am sure the pros would like a wider tyre in dry races, but it's not unheard of some running 30c tyres for the muddier races (like WVA has been speculated to have done in yesterday mud fest in Dendermonde). Keep in mind that narrower tyre also means less resistance (pushing a narrower section through a dense medium)
@Netakure yes, you get less cushioning from 33c than from 40/50c gravel tyres, but that extra precaution in choosing your line is a small price to pay for being able to actually ride through inches of mud.
They're better at not clogging up my seat stays...