There are two basic shapes of bike frame: frames with compact geometry and those with traditional geometry, also known as ‘horizontal top tube’.
Horizontal top tubes used to be found on all bike frames, with the exception of step-through frames. In recent years, manufacturers started making frames with top tubes that slope down from the head tube to the seat tube, creating a smaller front and rear triangle. This design is known as a compact frame, or compact geometry, and it requires less material to manufacture. The advantage to the cyclist is that a compact frame will be slightly lighter, at a given frame size. The advantage to the bike manufacturer is that they can specify fewer frames sizes in their range, since a compact frame allows a wider range of saddle heights.
However, transport cycling often involves carrying things on the bike. A particularly convenient place to stow D locks and water bottles, is the empty space inside the frame’s front triangle. A disadvantage of compact geometry is the reduced space for these items.
This picture shows Ernie Buck who circumnavigated the globe by bicycle. This is the bike he used. The frame has traditional geometry and the space inside the front triangle is being used to store two bottle racks, a D-lock, a cable lock and a frame bag.
Another advantage of a horizontal top tube is that the bike can be lifted, placed on the shoulder and carried with just one hand supporting it. This is useful for carrying it up steps or when walking over really rough or muddy ground. When carried by its top tube in this way, a bike with traditional geometry will be balanced whereas one with compact geometry will seem front heavy because the nose will point downwards.