Apple's own published specifications for the iPhone X say that it weighs 174 grams, but with the footnote that "Size and weight vary by configuration and manufacturing process."
This begs a couple of questions:
1. Has anyone noticed a size difference? I can't imagine there would actually be any size difference. That might make one model more difficult to fit into cases than another. Unless there are almost imperceptible size differences between space gray and silver because of a difference in materials (I would expect this to impact weight more though).
2. Has anyone used a scale to compare the weight between 64 vs 256 models, silver vs space gray, or between AT&T and Verizon models? Actual numbers to counter or confirm perception?
One thing is for sure: A number of people have described the iPhone X as a heavy smartphone. Going from an iPhone 6 to the X, I honestly didn't think of the iPhone X as heavy. I thought it felt a bit more solid, and of course it felt heavier than my 6... but not in a burdensome way. My wife, who also has an iPhone 6, said she was surprised at how heavy the iPhone X feels.
If anything could account for a weight difference, I would think it would be a difference in battery capacity. The iFixit teardown found that the iPhone X is the first to pack two batteries into the phone, and together they provide more battery capacity than even the 8 Plus, but less than the Galaxy Note 8. Of course the 64 GB flash memory module is going to be a different model than the 256 GB flash memory, but the weight difference should be slight. Perhaps the 256 comes with slightly more battery capacity? The iFixit teardown uses a 64 GB model.
Whatever the actual differences, it's hard for me to imagine someone returning an iPhone X because it's too heavy. I think it probably takes very little time to get used to the shape and weight even if you are coming from a much smaller and lighter model.