they use a microcontroller in that design - which is overkill to say the least especially when they're already using 3 transistors!
with mine a majority of the space is taken up by the relay so if you find a smaller relay then it'll be smaller however it's only the size of a 9v battery as it is - this was my goal cause most pedal boards of powered via DC adapters so it can be stuck in the battery compartment
http://www.paulinthelab.com/2013/04/soft-latch-true-bypass-relay-switch-for.html
www.paulinthelab.com - Stripboard Layouts