Price labels are drawn using the font size selected for the scale. The scale font defaults to "small" by default. To view the font size for any scale, double-click in the price scale area at the right of the pane. The font size options are small, medium, or large.
If you would prefer that all price labels be drawn in small font, regardless of the scale font setting, go to Setup > Configuration and set the variable
LargePriceLabels=false;