Why smaller allies invest in support infrastructure

Does it make sense for a small nation to build infrastructure for a larger ally's wartime forces?

Yes, when that ally is expected to operate in the region, because local support infrastructure gives its forces a practical base.

Infrastructure investment makes sense when a smaller country expects allied forces to operate nearby. Ports and support facilities give those incoming forces somewhere from which to work. The value depends on a realistic expectation that the larger ally will deploy into the area. Basing and sustainment capacity can be an alliance contribution in its own right, separate from matching the ally ship for ship.