I agree about those finger-tip toothbrushes. Can't get lighter, but not very convenient. And I worry about shoving my whole finger into my mouth 3-4 times a day. Sure, I'd try to wash my hands well before hand, but any sanitation mistake can really wreck your trip.
I'm liking these:
10 cm / 4" when extended. The brush end inverts and stores more compactly into the handle. The bristles are treated to resist bacterial growth on the bristles. That and a super-mini tube of toothpaste (or none) from a Chinese hotel and it's a very light set-up. 2 grams for the brush (0.07 oz) and 6 grams (0.21 oz) on the toothpaste but it gets lighter as you go. 8 for $4.75.