While I think that her reasoning here is sound, it has an underlying assumption that the size of the world population is unimportant. From my perspective I think that we humans are better off with the largest population our planet can support. Have you noticed that people who live in populous areas (i.e. cities) tend to be (generally) wealthier than those who live in sparsely populated areas? The more people we come into contact with, the greater the chance that their innovations will inspire innovations from us, and vice versa. Human technological innovation, which has played a central role in our efforts to improve our material lives occurs so much faster when we have more ideas from more people from which to draw.
Central to supporting an increasing worldwide human population is the production of sufficient quantities of food. Pre-1960s naturally occurring famine was not an uncommon event, today it is almost exclusively the result of political events (i.e. tyrants like Ethiopian "President" Mengistu Haile Mariam who deliberately starved large numbers of their people as part of a military struggle against the Oromo Liberation Front and other anti-Derg groups.)
What has changed things between then and now was an incredible series of innovations (beginning in the 1940s, yielding results by the 1960s):
- developing high-yield grains
- improvements in irrigation techniques
- modernizing land management techniques
- development and distribution of hybridized seeds
- development and use of synthetic fertilizers and pesticides
So my thought process here is:
- I think that increasing our standard of living is a good thing.
- Technological innovation is the way we have historically increased our standard of living.
- An larger population of well connected people is necessary to bring about that technological innovation.
- Modern farming techniques are necessary to support even our current population.
- Moving to a vegetarian diet will enable the feeding of a larger population than an omnivorous diet will.