I presume you mean the air is more dense in winter due to colder temps. However, I'm not buying the wintertime lean-out theory for our fuel injected engine with hot-wire MAF because the MAF will correctly measure the more dense air (colder temps will cool the "hot-wire" more, yielding a correctly higher reading). If the air is properly measured, the ECU will add more fuel to match more air. This means perhaps more power in winter, but not an induced lean condition.