As Laserbee said, you need a power meter to confirm the real power output of a laser.
Comparing one laser to another does not work really well, and usually not at all if they are different wavelengths. Something like a 5 mW green might look about as bright as a 200 mW red, if that red is 660-670 nm or so. If it is 635 nm the red will look much brighter despite having no more power output.
Camera's often make a mess of it too, especially when set to the default 'auto white balance' where they just try to figure out how to make a photo out of two monochromatic light sources. Camera's also tend to be much more sensitive to longer wavelengths that human eyes are: 1 mW of 635 light appear MUCH brighter to your eyes than 1 mw of 670 nm light. To a camera the difference could be pretty small.
This does not matter since camera's are generally used to take photos of scenery, people and such, and monochromatic light sources in nature are very rare.
Especially on the edges of the visible spectrum cameras sometimes show things too brighty. They can render borderline invisible wavelengths like 400 nm quite visibly, but also show long wavelengths in odd manners. Take a picture of a bbq with your phone: chances are that the coals appear some odd purple color while they are only dull red to your eyes.