My shoes had been pretty tight, to the point of discomfort, but the size up was too loose. I opted for tighter to reduce the amount of movement of my foot within the shoe, but it was a bit painful at the end of long rides in the summer, when the heat made my feet swell.
At the 2016 Cat 3 State Championships the skies opened up and the peloton got drenched (and they cancelled the race). By the time I got back to the car I realized my feet felt great. My shoes FIT great.
EUREKA!!! The rain had moistened the leather to the degree that it stretched. Ever since then I spray my shoes either in the sink or with the hose before I leave for a ride, and my shoes fit perfectly.
A few months later I found out this used to be common when everyone wore leather cycling shoes WAAAAY back in the day, before Facebook and smart phones. I found it while reading Steve Tilford's blog (article here), and here's the part that interested me:
All the shoes, up until this point were black. We would get them super small and carry a water bottle full of water and alcohol to spray them over and over so they would stretch to fit. It you didn’t do this, when you rode them in the rain, the shoes would get larger, super sloppy. Our shoes fit like tight gloves.
So if your shoes aren't fitting perfectly, try hosing them down and seeing if that does the trick!