What concerns does Jim Gallien have regarding Alex"s backpack?

From the text:

Alex’s backpack looked as though it weighed only twenty-five or thirty pounds, which struck Gallien—an accomplished hunter and woodsman—as an improbably light load for a stay of several months in the back-country, especially so early in the spring. “He wasn’t carrying anywhere near as much food and gear as you’d expect a guy to be carrying for that kind of trip,” Gallien recalls.