The additional weight provides more cushioning during falls, the researchers say in a study published Wednesday in the Journal of the American Medical Association.
"It's one of the interesting hypotheses," lead author Bill Leslie told the Toronto Star. "Individuals who are underweight ... don't have that padding."
Leslie, a professor of medicine at the University of Manitoba, said the decline in fractures began before the widespread availability of bone-density testing or drugs treating osteoporosis.
Since 1985, the hip-fracture rate across Canada declined about 32 percent in women and 25 percent in men, with the biggest drop from 1996-2005, the study found.
The rates dropped most among those ages 55-64.
The researchers didn't provide any corresponding figures for increases in weight or buttocks widths but did note weight problems and obesity are epidemic these days.
Better osteoporosis diagnosis, drug treatments and fall-prevention techniques also have helped reduce fractures, Leslie said. A drop in smoking, which is linked to osteoporosis, might also help explain the decline in hip fractures, he said.
The researchers stressed the study does not advocate obesity as a hip-protection strategy.
"Hip fractures continue to exert major effects on the population, and the decreasing incidence rates are not grounds for complacency toward osteoporosis prevention and treatment," the authors said in a statement.