How consistency helps your UI feel more designed
Simplify your interface font sizing.
When you are designing something and it doesn't feel quite right, step back and ask yourself "what can I make more consistent?".
Often new designers try to use just font size alone to differentiate between user interface elements.
Try reducing the number of font sizes you use, and use colour and weight instead - elements of your design will fit together better and feel more designed.
It's OK to vary font sizes if you have to, but do it with purpose.
Determine what it is you want to emphasise in your designs, and consider more than just font size to highlight elements.