Was your chicken fresh or frozen before steaming? Frozen often has water added (so part of the "chicken" you are paying for is actually water/ice) so they can sell it more cheaply and make a profit. If it was frozen initially, I'd use the "stewed" value for the weight after steaming.

You can calculate how much water was added to increase the frozen package weight if they state the amount or percent of protein in it (often stated as a minimum). You'll find that the % protein in frozen chicken is lower than Netzer's or the USDA's values for fresh chicken. Fresh raw chicken has about 22g protein per 100g, whereas frozen may be 15% or 18% protein (15g + 7g water & 18g + 5g water per 100g, respectively). When you steam it, that water/ice you paid a premium for will melt/evaporate.