Had the Brits not migrated to America, would England have such high population as US?

What's the reason for the first sentence?

No. It took, en masse, many of Britain's disadvantaged, as well as transported criminals and, significantly, its most entrenched superstitious people (Puritans and other non-conformists). The legacy for the USA can be seen in its (by western standards) high levels of religious adherence and superstitious belief leading to backward-thinking morality in areas such as abortion law.

Had the Brits not migrated to America, would we have had such a massive population as America?
I understand that not all Americans are from England, but those who are are estimated to be around 70 million. Would that mean the population of Britain would have risen to 130-140 million? I understand that population density and scarcity of resources puts a limit on population growth, but how much would the population have increased if the Brits never migrated to America do you reckon?

Well, the main reason why the US and Canada are doing far, far better than any other nations in The Americas, is due to England, and eventually the British Empire, encouraging huge amounts of their own people to settle over there and taking many Black slaves over there, whilst Portugal and Spain really only went there for gold and silver and took it all back home, while the British colonisers were using the land's resources to create long-lasting settlements there, like Virginia, Maryland, Massachusetts, North and South Carolina, Georgia, etc. (Not New York or Pennsylvania, though, as the European settlements there were made by the Dutch, not the English.)

If they didn't go, then England might have not become the British Empire it did become, maybe the French would've taken the stand instead, even though most of the colonisation they did in North America was trade posts, except for the city of Quebec and New Orleans, but they seem more likely than the Spanish or the Portuguese.

The puritans in actuality only occupied a few colonies. Their relevance is limited to a symbolic one. The real legacy of America's evolution is of opportunists, gamblers, and rebels. The emigrants who truly shaped America's destiny risked everything with no guarantees. They counted on nothing but their own potential and resilience. Imagine an entire continent encumbered with a social system more focused on class than any of us today can truly appreciate. Now consider the amount of untapped potential that must have existed in a perpetual state of frustration. America's colonization and subsequent mass emigration exploited that potential. "Give me you poor, your tired your huddled masses" is a nice sentiment but it is not entirely honest. It actually should have been, "Give me your desperate, your ambitious, your defiant masses." The early emigrants to the United States were bold people. They often spent their life savings on the ticket. They depended on no entitlements. They uprooted their families and faced an uncertain future. They were motivated by available land and ample opportunity, and that belief was founded in the conviction that a chance is all they needed. That kind of courage, and determination is uncommon amongst the privileged citizenship of today's developed world, and I fear that their spirit is lost to us. It is a shame because their place in history is significant.

The state with the highest % of English ancestry is, believe it or not, is Utah. I always found that an interesting fact.