Twice the size, but they don’t have anywhere near as much military hardware. Another factor, the US military logistics system is also the most robust in the world.

Infantry might fight wars, but logistics wins wars. (This is why Russia is losing in Ukraine, they lack the logistics system to deliver things outside their own borders)

China doesn’t have a great worldwide logistics system, but like Russia, they have a great internal system.

So a Chinese invasion of the US would basically be dead in the water, likely literally. As in, the ships carrying the invasion force would be sunk before they could unload.

A US invasion of China would be a deadly slog. The US would likely have air superiority, but that only goes so far.

The major factor in any fight, oil. China doesn’t have much in the way of their own reserves. They get most of their oil via tanker from the Middle East. And those shipping lanes are a massive weak point.

One that everyone knows, and also watches.

The final factor is the nuclear capabilities. China is outclassed there as well, but any nuclear fight is one that everyone loses.

The TLDR; China has a lot of troops, but not as much of everything else.