Yes, the weight does increase. However, most of the time, the listed weight of the denim is the post-soak weight, not the pre-soak weight. There are many exceptions, though, it depends on the brand and specific model of the denim in question.
Here are some examples of how these listed weights can vary:
1. Samurai's 21 oz. denim is a post-soak weight, but their 24 oz. denim is a pre-soak weight. Therefore, the 21 oz. denim is 21 oz. after a soak, but the 24 oz. denim is about 27 oz. after a soak.
2. Studio D'Artisan's 23 oz. denim is a presoak weight, so it is actually 25-26 oz. after a soak.
3. On Iron Heart's Anniversary model, both the pre-soak and post-soak weight are listed. The denim is marketed as 21/23 oz.
Also, when unsanforized denim is soaked, it gains close to 10% in weight. So in your example, the denim that is 12 oz. pre-soak would likely be closer to 13 or 13.5 oz. post-soak than 12.5 oz.
I find these weight details interesting to know, but like whodini said, when all is said and done, none of it makes that much of a difference.
Edited by commodorewheeler - 11/2/11 at 9:32pm