The water will add weight, and is the fist thing that would lower the actual total weight. Sometimes recipes called for drained weights, but its difficult to know what your using as to tomatoes. Some tomatoes are very pulpy and low in water, while others are quite watery, seedy and have little pulp. Sometimes if the recipe calls for plum tomatoes, you can expect a bit less water weight loss, compared to a recipe where you decide to use big beefsteaks that are very high in water weight. When I do tomatoes in the Villaware/Roma I cut the tomatoes open and scoop ou seeds and liquid, so the strained tomatoes are much thicker. I also use mostlt plum, sauce, or paste tomatoes, as they all tend to offer more pulp (mass) compared to a salad or sandwich type tomato. Suggest that you weight the tomatoes first, then weigh the amount after its been deseeded and skinned to see what a difference you would expect. Even for that, ist very hard to estimate what a drained weight would be if watery tomatoes were used, when a recipe calls for sauce tomatoes. In any case, if you ended up with just a few ounces lower in weight after straining, you can realize that the tomatoes are low in water and quite pulpy. If teh end product weighed have its weight after straining, it may be too low to expect a dsecent amount of product. Adjust any seasonings to taste. With most any recipe, unless your measuring the sauce with a measuring cup as a liquid, your not going to get accurate or reliable results if you go by weight. In most cases weight measures are used for dried ingredients as they can vary grealy depening on partical size, packing into a cup, and if there is any trapped air, like in the case of bread crumbs or cubed bread. Here, I only use a recipe as a guide to tell me what is needed in the end product, and don't usually measure out the amounts. A good example is a big bunch of frozen raspberries to extract juice. It may call for 3 cups of berries, but after removing seeds, you ed up with just 1.5 cups of juice. I would concentrate on the last measurement as oppsoed to the inital one.

Susan,
If you weigh, then measure them for the right amount before freezing, use the whole container in the recipe when you make it and it will be fine. Some of the cell walls will break down and collapse during freezing, but you started out with the right amount. It doesn't disappear just because you froze it.
I always do this when freezing things for canning. I do juice ahead and freeze in the right amounts for jelly or fruit for jams. I do peppers and onions for salsa, etc, too.

Ok, thanks. I will do that.