Actually, it's just the fact that tires don't dig in to provide traction in LFS, since the grass or gravel is infinitely stiff like tarmac. IRL 4wding, it's well known that a large diameter and moderately wide tire grips better overall than stupendously wide but small diameter tires. Factor in the fact that aggressive tread patterns literally dig into the relatively compliant gorund, the tires could practically "pedal" their way through soft ground. The optimal width for any given tire diameter is entirely dependent on the terrain stiffness. For VERY soft mud or sand, wider is always better, since low ground pressure is crucial to avoid excessive sinking. However, your typical beach sand is best done with moderately wide tires at realtively low (~15psi) pressures.
On the other hand, snow requires relatively narrow tires to provide enough pressure for the sipes to dig in. That's why WRC cars use narrow siped tires on the Scandinavian snow stages.
To sum it all up, it's basically a tradeoff between the tire's propensity to dig in for traction with the need to avoid excessive sinking to achieve the optimum tire package. The problem isn't of LFS's tire physics being fundamentally wrong, it's just the lacks of the effects of a compliant ground surfaces on tire grip.