The strength of your votes on LessWrong depends on your karma; if you have more karma, your upvotes will increase the score of things you upvote by more.

Normal votes (one click) are worth

2 points – if you have 1,000 karma
1 point – if you have 0 karma

Strong Votes (click and hold) are worth

16 points (maximum) – if you have 500,000 karma
15 points – 250,000
14 points – 175,000
13 points – 100,000
12 points – 75,000
11 points – 50,000
10 points – 25,000
9 points – 10,000
8 points – 5,000
7 points – 2,500
6 points – 1,000
5 points – 500
4 points – 250
3 points – 100
2 points – 10
1 point – 0