English Unification happens way slower, or is averted, Countries on the British Isles try to stop Britain from overpowering them, Slowing down formation of Britain as a country. Britain formed in 1740s, instead of 1700s. Jamestown founded in 1620, instead of 1607, and natives kill it early. 2nd Jamestown in 1635, that continues much slower. English Civil war lasts WAYYY longer, killing alot more people before it ends. Then have it proceed stright to the Glorious Revolution, and make it last really long. Annexation of Ireland alot harder.
All this causes Britain to have a lower population , and makes it harder to mantain colonies. They thus grow slower, and expand slower.
Eventually, USA secedes, with roughly the same as OTL territory, but a lower population. The fight goes on and doesn't end well for the US. USA breaks up into USA and CSA, leaving two rump states that fail to expand,largely because of continual fighting with each other. Eventually, Canada becomes dominant on North America.
US is basically West Virginia, Virginia, Delaware,New York,Rhode Island, Washington, New Jersey, Connecticut, New Hampshire,Maine, Vermont,Maryland and Pennsylvania.