Nah it is not. According to the website it is 247g which is good, about half the weight of a rubber 36" tube, but it is not as light as a 29er- I weighed one at 178g. If I were you I would stick with what is working- if something works don’t fix it!
I’ve had trouble with my Foss tube in my 20" Equinox that I bought late last year. I pumped it up to 80psi a few times after about two or three days it would always be back at 40psi. The sidewall is rated 85 to 110psi. One day about a month ago I pumped it up to 90psi and the next day it was flat. I discovered a hole in the seam - a manufacturing defect.
I tried a patch on the hole, the special Foss ones and put a small amount of air in. The air crept out along the seam and under the non-vulcanised patch. I tried a lighter- encouraged by the videos on youtube of people sticking nails through their Foss tubes and fixing them by melting it with a lighter and stamping it shut. It made it worse as it seemed to melt the glue holding the seam. I tried again with a patch but I sanded the seam down so it was a nicer surface. Again the patch did not stick. I went back to an old rubber tube and patched the slow leak. I found several other people on Bike Forums who had problems including seam failures with them and gave up. Others swear by them so maybe I should try again.
I reckon good luck on the 29er tube!
Where do you get your figures? I used scales. Stretching isn’t that hard if you are careful and patient. You must be getting really heavy 29er tubes, what brand?