Low voltage cable lighting - voltage drop & resistance

I am putting up cable lighting and have a question about voltage drop and resistance. My cable run is about 33 feet with seven 20w MR-16 fixtures. I am using a 12v transformer that with a max load of 140 watts. When I connect the lights they are not bright at all.

I accidently (it's always and accident) destroyed the cable that came with the fixture and bought coated wire rope at the hardware store. The actual wire cable without the coating measures 3/32" which translates into a wire guage of between 12 and 10. Do I need a larger guage wire? I am a bit confused because I read something on the net regarding low voltage landscape lighting that was saying that using a smaller guage wire increased the "resistance" and therefore current was able to reach the end of a long run.

Also, is it best to fully use the available wattage? Any help will be greatly appreciated.

Regards,
George, SF, CA