The question gets it backward: Manitoba fits into England—not the other way around. Let's look at the numbers.
Manitoba is nearly five times larger than England. You could fit England inside Manitoba almost five times over with room to spare.
England feels "bigger" psychologically because of its population density (434 people/km² vs. Manitoba's 2.3), global cultural footprint, and historical weight. Manitoba's vast boreal forests, thousands of lakes, and subarctic expanses are sparsely inhabited—most of its 1.4 million residents cluster in the south near Winnipeg. England's 56 million people pack into a compact, intensely developed landscape.
If you overlaid England on Manitoba, it would occupy roughly the southern third of the province—from the U.S. border up to around Lake Winnipegosis. The rest of Manitoba (the Interlake, the North, Hudson Bay coastline) would remain entirely empty of England.
So the premise is inverted: Manitoba doesn't fit in England. England fits in Manitoba—almost five times.