You're right, it is an Americanism...
"Ballpark" has been used to mean a broad area of approximation or similarity, or a range within which comparison is possible; this usage OED dates to 1960. Another meaning, "sphere of activity or influence", is cited in 1963. "In the (right) ballpark", meaning "within reasonable bounds" dates to 1968. A "ballpark figure" or "ballpark estimate", one that is reasonably accurate, dates to 1967. (Source: The New Dickson Baseball Dictionary).
Although I'm not familiar with the size of an English Cricket Pitch, I think the American baseball park is considerably larger. The distance between bases is 90 feet but the distance from home plate to the left field wall is near 400 feet in most Pro ball parks. So, anything within that park occupies only a small part of a very large piece of real estate indeed. The phrase, as you've surmised, is a rough estimate or very generalized approximation...