That depends a LOT on the technology used - some technologies are much more efficient than other (converting a higher percentage of the power used into visible light).
Normally the watts is a measure of how many watts of electicity a bulb uses, so a 60 watt bulb uses 60 watts. The brightness is measured in lumens, so a 60-watt incandescent blub might produce 600 lumens while a high-efficiency fluorescent blub might produce 3000 lumens.
This depends on the type of bulb and it's efficiency. Incandescent bulbs produce 700-900 lumens at 60 watts.
Lumens measures how bright it is, watts measures how much electric power it uses up. An old-type incandescent bulb produces about 10 lumens per watt. A halogen produce about 13 lumens per watt. A fluorescent (energy saving) bulb produces about 50 lumens per watt. LEDs produce somewhere around the same as a fluorescent.
Original answer: Because it gives off more power. Updated answer: In general, because the 100-watt bulb produces more total light (lumens) because it also consumes more power than the 60-watt bulb. However, a 60-watt can produce more lumens than a 100-watt bulb, depending on the types of bulbs in question.
Bulb brightness is measured in lumens which express the amount of light produced, while the electric power used is measured in watts. The typical efficiency of some different types of bulb is: Incandescent 12 lumens per watt Halogen 17 lumens per watt CFL (compact fluorescent) 50-60 lumens/watt LED (light-emitting diodes) 90-100 lumens/watt.
In this context, Watts are a measure of electrical power and lumens a measure of light output. So, depending on the efficiency of your light source the equivalency will vary dramatically. For example, a 60 watt conventional light bulb typically gives off 800 lumens. A frosted 60 watt bulb may only give off 500 lumens. A 13 watt CFL will give off that same 800 lumens, which is why 13 watt CFLs are often (erroneously) called 60 watt bulbs.
Different technologies produce different amounts of light - measured in lumens - for a given amount of electric power - measured in watts. Incandescent: 12 lumens per watt Halogen: 16 lumens per watt CFLs: 50-60 lumens per watt LEDs: 100-120 lumens per watt
Bulb brightness in LumensA 100 watt Bulb is rated at approximately 1700 lumensA 60 watt incandescent bulb is rated at approximately 800 lumensA 40 watt bulb is rated at approximately 400 lumensA 25 watt bulb is rated at approximately 180 lumensA 4 watt night light bulb is rated at approximately 20 lumens
10-11 lumens per watt for incandescent bulbs 13-14 lumens per watt for halogen 50-60 lumens per watt for fluorescent
About 50-60, which is 4-5 times better than incandescent bulbs.
If the two bulbs use the same technology the 100 w bulb is 10/6 times brighter than the 60 w. Incandescent bulbs give about 10-12 lumens per watt Halgogen gives about 15-18 lumens per watt CFL (low-energy) gives about 50 lumens per watt.
Compact fluorescent bulbs are widely used because they use one fifth of the electric power to provide the same amount of light as an incandescent bulb. Incandescent: 12 lumens per watt Halogen: 15 lumens per watt CFL: 50-60 lumens per watt LED: 100 lumens per watt High-pressure sodium: 200 lumens per watt