So of course we all know that when we zoom in on something, it appears to get bigger and takes up more of the viewfinder/camera sensor. However, I believe this question is asking about what is called magnification factor.
You most commonly see on this macro lenses where the whole point is to enlarge something that in real life is very small. The magnification is described like so:
original object size in real life:resulting object size on the sensor
So a lens with a 1:1 magnification factor would take an object that is 10mm across and project an image on the camera's sensor that is also 10mm long. A 1:2 factor would convert 10mm to 20mm. 1:3, 10mm to 30mm and so on.
(Note that a magnification factor of 1:3 can also be written as 3.0x. Also this factor is for a given minumum focusing distance. At other distances it won't have the same effect.)
A 1:1 magnification factor may not sound too great but when you think that today's camera's have on average about 18MP or so you can do quite a lot.
So why would two different lenses with the same focal length have different magnification factors? It really just ends up being differences in build quality and the actual purpose of the lens.
Feel free to comment. Hope this helps.