The old school looks smaller partly because the person looking has changed.

Places do not shrink; the measuring instrument changes. Corridors that felt endless were crossed by a smaller body with far less experience of buildings, and returning as an adult brings a different set of proportions into the room. The flatness people feel is really a report on how far they have come. What altered is the person doing the looking.

If you go back somewhere from childhood, expect the shrinking and let it be interesting rather than sad. Try recalling one specific thing that happened there instead of judging the size. Take it as evidence of growth in areas you cannot measure as easily.