The smaller size of the wire means that the resistance of the wire is higher. Electrical theory tells us that the greater the resistance, the lower the output will be at a give input level. Simply stated, the wire resists the flow of electrical signal. That is why you noticed a difference in sound levels between your test speaker and the other speaker.

The microphone cable is made for a different purpose, and has the manufacturing efforts and materials directed to that goal. You could run 2 pieces of microphone cable, 1 for positive and 1 for negative to the speaker and test again. This would effectively lower the resistance by using both conductors for each channel.

I would still look for a dealer that will allow you to audition cables.