most cross frames have horiz top tubes to create more space making it easier to get the bike on your shoulder.
for racing, the frame should be as close as possible to your road frame, especially the top-tube length. for general off road use, a size smaller is common, but not more than 2cm otherwise the bike will feel cramped
EDIT: D’oh should have typed quicker.
modern cross isn’t that conservative either…. (caution, willy waving)