Most headphones are designed for average-to-large heads. If yours is smaller, they'll slip forward, feel loose, and look oversized.
The key metrics: headband adjustability range (how small can it go?), clamping force (enough to stay put without being tight), weight (lighter = less likely to slide), and earcup proportions.
The Sony WH-1000XM5 has a highly adjustable headband that accommodates smaller-than-average heads well. The moderate clamp force keeps them in place during head movement. The earcups are relatively compact compared to the Bose QC Ultra.
Weight is underappreciated for small-head comfort. The Sony WH-CH720N at 192g is the lightest ANC headphone Sony makes. Less weight means less downward pull and less sliding.
On-ear headphones sometimes work better for small heads because they sit on the ear rather than around it, creating a smaller overall profile. However, they sacrifice comfort and isolation compared to over-ear.
