Changing your track from large to small and then back to large again is known as necking or necking down. This is often required when you have to go between integrated circuits or component pads. You can have nice big low impedance tracks with enough flexibility to route between tight spots.
In practice, your track width is designed according to the current flowing through it and the maximum temperature rise you are willing to tolerate. Every track will have a certain amount of resistance, so the track will dissipate heat just like a resistor; the wider the track, the lower its resistance.
The calculations to figure out a required track width based on the current and the maximum temperature rise are a little complex. As a rule of thumb, a 10° Celsius temperature rise in your track is a nice safe limit to design around. In addition, the wider the track, the better.