Difference on component size (Resistor, Capacitor, inductor)?
Couple of reasons may come into play:


This a prototype board where the volume is low enough that it's cheaper to hand assemble instead of machine assemble.  In this case, having bigger parts where they can fit makes assembly easier.
Some parts are being used where their parasitic values come into play.  The 603 might have a bigger parasitic capacitance or inductance over the 402, which can be undesirable (or desirable) in some situations, especially at higher frequencies (100s of mhz / ghz).
The engineer who did they layout goofed, or blindly copy-pasted.


Without seeing the board and the application, it's hard to tell.