Depends on the type of insulation you're talking about :D
For down the loft (given in inches) tells you how thick the layer of down. The more loft, the more insulation you'll have. Fill power refers to the volume, in cubic inches, one ounce of down occupies. So one ounce of 900 FP down fills 900 cubic inches. Obviously, the higher the FP the less down you'll need to fill a given volume.
For synthetic the unit you usually see is grams per square meter. In this case the heavier the material, the warmer you'll be (generally speaking). For example, the Patagonia Nanopuff uses 60g/m2 insulation while the Micropuff 100g/m2 insulation. So you'd expect the Micropuff to keep you warmer than the Nanopuff. But much like down, not all synthetic insulations provide the same insulative properties.
Of course, there are numerous other variables that affect how warm you will feel.