When we talk about the weight of a gem we have two things in mind, how big is it in appearance and what does it weigh. 

Size has to do with a gemstones dimensions, and weight is just that.  In the gem world we weigh gems in the metric weight of carats which is 0.200 grams or a 1/5 of a gram.  A 5 carat diamond or gem weighs 1 gram.  The carat is also broken down into points a point is 1/100 of a carat, hence 1/4 ct = .25 ct, 1/2 ct = .50 ct, 3/4 ct= .75 ct and it could be 25/100, 50/100, 75/100 and so on.  Most of the time though you will see weight in this form, .25ct, .50ct, .75 ct and 1.58ct. 

Diamond weight or for that matter any gemstone's weight is based on its S.G.

Specific Gravity is the ratio of the density of any substance to that of water at 4 degrees c.  The S.G. of gems is usually obtained by hydrostatic weighing*.

What this means to you is that gemstones that are  the same material and have like size will weigh close to the same, (as an example most 1 ct. diamonds have a diameter of app. 6.5mm with a 3.9 to 4 mm depth).  If this is true, stones that are less than 6.5mm in diameter but still weigh 1ct must have a greater depth percentage than normal.  The reverse is also true, if the diamond has a greater diameter than 6.5mm  then the stone must be under the proper depth percentage to be 1 ct.

As noted above, size in relationship to weight can be deceiving.  However specific size or dimensions will give an approximate weight.  Please refer to the weight chart.

*Hydrostatic weighing is the weighing of a substance first in air, then in water.   The S.G. is then obtained by dividing the weight in air by the difference between the weights. The specific gravity weight is used more for separating one gem material from a look alike, but it also explains why a smaller gems can weigh more than a larger one.