You do realize that the entire current population of Earth, if each one had the same space as a small house, could fit into a land area the size of Texas don't you?
But it takes significantly more land than that to support a First World lifestyle for each of those individuals. Agriculture, energy production, transportation, manufacturing, etc? As it stands, if everyone in the world today lived the typical lifestyle of a First Worlder, there would need to be two or three Earths. (Seriously,
check it out.)
Since, obviously, no one wants to live a lifestyle with lower living standards than that of a First Worlder, and since population growth is inversely related to education levels -- and since World War III and the resulting indirect deaths would carry a very, very large body count -- the logical presumption is that Earth's population would probably be decreased by at least a third by the time of the 24th Century.
Overpopulation worries have been around for years. Earths population has doubled in that time and there are fewer people who starve around the world now than then.
Evidence? Last I checked, something like a third of the world's population lives on less than a dollar a day. Thousands of people die of starvation and malnutrition every day because of extreme poverty.
With the technology shown in Star Trek, the Earth could easily sustain a population in the 25 to 40 billion range and still be quite nice.
Maybe it could. On the other hand, many warp reactors aren't allowed within planetary atmospheres for fear of what the accidental release of that amount of energy could do to the ecosystem and to human populations. We don't know. But we
do know that overpopulation tends to be bad for any species, we
do know that the lower a population is, the further its resources can go for each individual, and we
do know that people tend to reproduce less when they have access to greater education and to contraception. A lower population is just
better for everyone involved -- that doesn't mean that people should kill each other, of course, it just means that people shouldn't have huge families. There's really no need to go beyond maintaining replacement-level growth, if that.