The head sizes of babies are actually getting larger over the past decades [1]. This might be because of better nutrition, but maybe it is by a large part because we can do caesarean sections in every hospital now and head size is not as limited as it was before anymore.

[1]: not the best source, but one I could find now: https://www.tandfonline.com/doi/full/10.3109/07853890.2011.5...