And here is your list of summer Olympic sports (we'll compare cross country to other sports in the summer Olympics since summer is where cross country would go if it were an Olympic sport):
* Archery
* Athletics
* Badminton
* Baseball
* Basketball
* Boxing
* Canoeing
* Cycling
* Diving
* Equestrian
* Fencing
* Field Hockey
* Football (Soccer)
* Gymnastics
* Handball
* Judo
* Modern Pentathlon
* Rowing
* Sailing
* Shooting
* Softball
* Swimming
* Synchronized Swimming
* Table Tennis
* Taekwondo
* Tennis
* Triathlon
* Volleyball
* Water Polo
* Weightlifting
* Wrestling
Archery - cross country is probably more widely competed than archery
Athletics - cross country is a subdivision of athletics
Badminton - cross country wins unless you count playing in your backyard and at family reunions
Baseball - in the US, baseball probably wins, on a world level, cross country probably wins
Basketball - cross country loses
Boxing - in the US, cross country is much more common, this probably extends to the world level as well
Canoeing - excluding recreational canoeing, cross country wins easily
Cycling - I'm guessing at least forms of biking have more widespread competition than cross country, but certainly not velodrome racing.
Diving - easily more competition in cross country. Diving is worldwide, but too gymnastic in its roots to have large participation anywhere
Equestrian - if you don't count every person who ever rode a horse, you'll find far more people competing in cross country worldwide
Fencing - you're kidding me, right?
Field Hockey - this hoity-toity sport has far less worldwide competition than cross country
Soccer - okay, this one dominates cross country
Gymnastics - are we talking eight-year-old girls or athletes in their early 20s?
Handball - on a serious, organized level, far more competition in cross country
Judo - I don't have a good read on this one, especially outside of the US
Modern Pentathlon - incorporates cross country, and no way its bigger than cross country
Rowing - not a chance its bigger than cross country, too exclusive
Sailing - ditto rowing, only more so
Shooting - recreationally, probably bigger than cross country, but not on an organized, competitive level
Softball - big in US, not so much elsewhere as evidenced by the trend of Olympic medals
Swimming - the whole is bigger than cross country
Synchronized Swimming - heh, heh
Table Tennis - maybe bigger in many nations, not sure how it would compare on an international level
Taekwondo - see Judo
Tennis - recreational or competitive and organized? If the latter, cross country is bigger
Triathlon - a great sport for habitually injured runners with disposable income
Volleyball - maybe bigger than cross country considered as a whole, but the parts (and especially on the men's side) are not as big on an organized level
Water Polo - probably not as big as cross country on an international level, at least partly due to expense of monopolizing a pool
Weighlifting - Outside of the Olympics, how often do you see people competing in snatch and clean-and-jerk?
Wrestling - I would guess this one is bigger on the international level--very big in eastern Europe and Asia, respectably big in North America.