I'm a bit surprised to see Penn Station below Grand Central.
Penn Station is the most trafficked train station in North America[0], which I would imagine would lead to more subway entrances/exits, especially during rush hours.
Also, Penn Station has the A,C,E, 1, 2, and 3. Grand Central only has the 4/5/6[1]. The 4/5/6 are the only lines on the east side and are therefore fairly busy, but I find this surprising nonetheless.
[0] This includes non-subway trains,
[1] Don't even get me started about the T (ie, the Second Ave. Line)! :)
Not every commuter takes the subway to and from Penn Station. The surrounding area has a lot of offices, so many people take light rail there and then just walk to and from work.
As others mention, Grand Central has the S and 7 (crosstown) which connect to Times Square.
Grand Central gets 800,000 visitors per day, getting many commuters from Long Island and Connecticut.
Grand Central is also a much bigger stopping point. It's near many more office buildings. Penn Station is in an urban area, and next to the Garden, but I don't think it's quite as dense. Many folks would exit at Times Square.
The 4/5/6 line is the busiest line in the country. It gets more traffic than the entirety of Washington's, Chicago's, Boston's and San Francisco's lines.
Penn Station is the most trafficked train station in North America[0], which I would imagine would lead to more subway entrances/exits, especially during rush hours.
Also, Penn Station has the A,C,E, 1, 2, and 3. Grand Central only has the 4/5/6[1]. The 4/5/6 are the only lines on the east side and are therefore fairly busy, but I find this surprising nonetheless.
[0] This includes non-subway trains, [1] Don't even get me started about the T (ie, the Second Ave. Line)! :)