Overcrowding prevents crisp for two different reasons. When baking, it’s because of trapped steam. This steam can come from the natural moisture of the potatoes, but it can also come from failing to dry them after soaking them in cold water. With nowhere else to go, the moisture will turn to steam when baking, and the fries will become soft. It’s for the same reason that the side of the fry touching the pan will usually be a lot less crispy. If you want to avoid this, in addition to spacing the fries out, make sure to either flip them halfway through baking or bake them on a wire rack.
When it comes to sweet potato fries that are fried in oil, overcrowding leads to soggy fries for another reason. It has less to do with the moisture, but rather the fact that adding more food to the oil causes its temperature to drop. When the oil isn’t hot enough, the sweet potatoes absorb more and take longer to cook, ultimately preventing them from crisping up.
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