Home prices keep rising .  One reason is that today’s houses are bigger: Buyers now want more rooms, washers and dryers and air-conditioning. But even if you adjust for the improvements, houses cost more today . Why? Politicians blame big companies like Blackstone, Invitation Homes and JPMorgan because they’ve been buying thousands of homes. “It’s troubling,” said Sen. Elizabeth Warren (D-Mass.), that homes are bought “not by families who want to live in them, but by speculators who want to use