Different sizes for different needs -[ HMM ]-

One small and two much smaller relays. The only things in common between both types is that they are single pole devices (similar to ordinary light switches) controlled by low-voltage direct current power sources.

 

Unlike the larger solid-state (non-mechanical) relay, the smaller ones employ a tiny reed switch which can handle up to 200 volts AC or DC, but at a rather minimal 0.5 amperes maximum current, and requires a fixed three-volt DC source to operate.

 

The larger relay is 2.25" (57.2mm) long; the smaller relays are 0.78" (19.8mm) long.

Done