She’s delusional! A Birkin 35 (a real one anyway) is heavy when empty. The basic cow and bull hides H uses are the best quality and inherently heavy, and whereas most bags will have a stiff cardboard in the base of the bags, H bags are constructed of several layers of leather, which add additional weight. The only exception is chèvre - goatskin, which is very lightweight, but 35’s in chèvre are extremely rare, as the goats don’t grow large enough, so the resulting hides are too small. I would say from just looking at the picture, her bag is probably Taurillon Clemence, a fairly thick, heavier leather that tends to “relax” a bit, as the front of hers has done. On the plus side, it’s very durable, can withstand getting wet and develops a nice patina with age. But, it is heavy. (Or, it could be Epsom, which is lighter, but it’s a stamped leather with an overall consistent grain that has been manufactured rather than occurring naturally.)
Regardless of what leather it is, she’s just trying to justify what she’s bought! Jane Birkin, the bag’s namesake, stopped carrying her Birkin 35, as she developed tendinitis and was told that the bag was to blame. She was very ticked off at Hermes! A Birkin 35 is heavy - there’s no way around it.
I’ll shut up now.