A mug looks like one object and is made as two. The body is thrown or cast; the handle is formed separately, often by pouring slip into a plaster mould that leaves it hollow. While both are still leather-hard, the ends of the handle are scored, slipped and pressed onto the cup. From that moment the two pieces have to dry and shrink together, or the joint will open.
Why hollow
A solid handle of that thickness would be heavy, but heaviness is the smaller problem. Clay shrinks as it dries and again as it fires, and a thick solid section shrinks unevenly: the outside stiffens first, the inside stays wet, and the difference shows up as a crack. A hollow handle has a thin, fairly even wall, so it dries at roughly the same rate as the cup wall it is attached to. Hollow is a drying decision before it is a comfort decision.
It also explains the tiny hole sometimes found near the join. A sealed hollow pocket traps air, and air expands in the kiln. A pinhole gives it somewhere to go. What looks like a defect is a vent.
The joint is the whole difficulty
The two ends of a handle meet the cup at different heights. Between them, the loop has to shrink by the same percentage as the wall between those two points, or one end will pull away. This is why a handle is shaped with the shrinkage already allowed for, and why the join is often smoothed with a wet tool rather than simply stuck on. In a tableware works in Chaozhou, the person attaching handles is doing the most exacting job on the line, not the most casual one.
Look under a mug and you will often find a bare ring of unglazed clay. The piece sat on that ring in the kiln; glaze would have fused it to the support. The same firing that gives the handle its strength also decides where the mug is allowed to be glazed.
Why the handle sits where it does
Position is a matter of balance and of packing. Too high and a full mug tips forward in the hand; too low and the fingers touch the hot wall. But the handle also has to survive a carton: mugs are stacked or nested in rows, and a loop that sticks out further than the rest sets the spacing for the whole box. A few millimetres of handle can change how many mugs fit on a kiln cart and in a shipping carton.
Reading one in the hand
Run a thumb along the join. A well-made handle has no hard line where it meets the wall, because the two were becoming one piece before the firing was finished. Turn it over and find the vent hole and the bare ring. Then lift it empty: the weight should be in the body, not in the loop. A mug is a record of two pieces of clay agreeing to shrink at the same rate.