Meat is a big one, but its the #3 - #5ish big one depending on how you look at things (at an individual westerner vs. other lenses). Roughly in order it goes - having more kids than adults (population growth) - living in a single-family detached home (land use sprawl + hvac inefficiency) - owning a car - flying - eating red meat (particularly beef & lamb) So yes by all means, eat less red meat, but after you've moved…
Good news on that point is that the number of children in the world already peaked several years ago. The continuing population growth is now being driven solely by life expectancy increases. Growth in lifespans in developed countries seems to be running out of steam, so once developing countries have caught up we would expect population growth to stop fairly soon afterwards.