Wouldn’t a smaller population consume less goods and power?

In the long run yes, however, (a) realistic population controls would likely mean a stable population, not a much smaller one, so that doesn't help much for deceasing consumption; and (b) even with totally unrealistic population controls (e.g. zero births worldwide) the population decrease would be slower than the time scale in which we need to reduce emissions - zero births would mean a population decrease of less than 1%/year (currently 60m deaths/year over 7.8 billion, the world skews young-ish so the death rates don't match what you'd expect from life expenctancy) but we need an emissions decrease of 1.5%-2%/year to keep the global warming in reasonable limits.

A smaller population would consume less goods and power, but since we need much less carbon emissions, achieving that through a population decrease alone would require a much smaller population i.e. so much extra death that IMHO that would be more horrible than the consequences of any climate change.