After reading your question, I think the difference between the required size of most of the lines on a typical sailboat and the topping lift come from the difference between static load and dynamic load. The load on a topping lift is mostly static. It must hold the boom in place. We can determine what this load would be, and can control it by how much torque we put into the mainsheet. It can be calculated fairly acurately. The loads which must be handled by sheets, vangs, reef lines,halyards are dynamic because they are required to counteract the forces of the wind on large areas and these loads can be massive and can be significantly augmented by other forces induced by movements within the boat such as gybes, pitching, rolling, etc. The end result is that as long as the topping lift is not suffering from damage or old age, it will do the job. Obviously, if you excessively crank on the mainsheet with a winch, you could break something but under normal use and kept in good repair, the topping lift should not be a problem. If it's still a big concern, get a ridgid vang. You probably knew all this, but sometimes a question causes one to ramble. Hope it helps, ROwen, Mikayla