Typically "bigger is better" is a safe bet but there are some things to consider.

Fat tires add drag so that will reduce mph.

Also if the tires are not matched properly you will get too much oversteer or too much understeer. There can be adjustments made to correct or reduce the o/u steer but if there is too much tire then it will make it difficult if not impossible and you will sacrifice performance in other areas.

And obviously if the tires are so fat they rub then you are asking for big trouble.

They also need to be correctly matched to the rim width. Incorrect tire/rim match reduces the sidewall stability.

Car setup is all about compromises.

Of course there positives to fat tires too, but everyone knows what those are.