Those hats, like a trilby hat or a bowler which often have a wide leather headband stitched inside can easily be adjusted smaller. This clever hat tip dates back a hundred years, when gents would adjust their smart trilby hats with a scrap from their morning paper:
1. Put the hat on, and feel where it is too big by checking with your finger at the front and side to see where the largest gaps are
2. Take a square of newspaper or tissue paper about 10cm wide and roll it up using 2-3cm folds
3. Place the rolled up paper carefully behind the headband at the position you noted needs 'filling out'
4. You may need to adjust the amount of paper and its position in order to find the perfect fit
5. Once perfectly adjusted, your hat will remain the perfect size forever
6. For a bit of fun, you can use a memorable text, page from a book or a personal photo instead of plain old newspaper. Some men used to write their address on the paper in case they ever lost their hat it could be found.