Filling the Frame
Composing so the subject occupies most or all of the picture area, leaving little surrounding context.
In depth
Filling the frame removes ambiguity about what the picture is of and forces the viewer into close engagement with texture, expression and detail. It is achieved by moving closer rather than by zooming, since the two produce different perspectives. The cost is context, so it suits subjects whose interest is intrinsic.
Examples in practice
Moving in until a weathered face fills the entire frame turns a portrait into a study of skin and expression.