High and/or rising populations are not a cause of rising wealth. There are many factors involved with wealth, but if you want a kind of overriding concept, it is Free Market. A free market is simply a market with little or no barriers to entry or exit for both buyers and producers. This contributes to innovation which contributes to rising efficiency and that means increasing wealth.
In this environment, increasing population is a benefit. One one side, it means a growing market for a producer's goods and services. On the other side, it means more availability of skill and labor. Whether as employee or entrepreneur, they are an important resource to the economy.
France's birth rate did not actually fall in the mid-18th century. I can find only documentation that the increase of the rate slowed significantly. In this, it preceded a similar decrease in rate for the rest of Europe by about 80 years. So France, for whatever reason, seems to be a substantial outlier here.
That the GDP increased is also hardly surprising. France's economy was suffering the double whammy of Louis XIV grandiose indifference and John Law's incompetence. It greatly expanded its trade, especially with the New World and thrived. For a while.
I cannot find any data that shows lower living standards in England after the Industrial Revolution. Sure, conditions were harsh, but two aspects reveal that this was still an improvement on conditions in the past. First, there was a movement of workers from farm to factory. This movement was voluntary. Second, after experiencing the harsh conditions of the factory, there was not a return to the farm. This tells me that, no matter how bad those conditions appear to us today, they were still better than the previous conditions.
Countries that were once poor but are now wealthy, did not achieve this result overnight. They went from extreme poverty to less extreme poverty to impoverished to modest wealth to affording a few luxuries, etc. Unfortunately, there is no short-cut.
Btw, concerns like worker safety and environmental concerns, things we tend to take for granted today, are actually luxury items. They become important only when there is enough wealth so that more pressing issues like, "when am I going to be able to feed my family again?" are no longer the major concern of the average worker.