If you're referring to the fact that they were 1.5" by 3.5", that wasn't shrinkage. That's nominal vs. Actual size. The lumber is indeed 2x4 at rough cut, but

If you're referring to the fact that they were 1.5" by 3.5", that wasn't shrinkage. That's nominal vs. Actual size. The lumber is indeed 2x4 at rough cut, but when you buy S4S (surfaced four sides) lumber, it's been surfaced down smooth. That process takes some material off, leaving you with the half-inch discrepancy.