Things work better in pairs!
Think it like this… You tie your left hand behind your back and you have this nice juicy steak in front of you, hard to cut it up in the smaller bites ;-} Even if you tied your arm to the side so you could still use your hand things are still not so easy.
While it may not seem to be the same RAM is similar! Which is one of the reasons Apple populates your system with two RAM modules and with rare exception so do other makers do as well in their newer systems.
Basically, this is how RAM memory is made to go faster! Interleaved memory.
So while your system should work in either configuration you are over stressing the one or larger DIMM and a second issue is the ability of the module to not overheat which is why two DIMM’s are also used as well.