to bear a heavy load: to take a weight onto your own shoulders or back and go on holding it up
This verb is about weight. It pictures a load hoisted onto the shoulders or the back and then held there, which is why it belongs first to porters, pack animals, and gangs of drafted labourers. Hebrew has a far more common verb for carrying (נָשָׂא), and that one can also mean to lift, to take away, even to forgive. This word keeps the load pressing down on whoever has taken it.
Isaiah sets the word's two directions side by side. In one verse worshippers hoist their idol onto a shoulder and haul it around, because a god of wood and metal cannot move itself. A few lines earlier the LORD had used the same verb of himself: he has carried his people since they were born and will still be carrying them when their hair is grey. In one case the thing carried is a helpless object; in the other it is a whole nation, and the sting is that only one of the two parties is doing any actual carrying.
From there the picture moves inward, because Hebrew treats trouble and guilt as weights too. A ruined generation says it is carrying the wrongs its fathers committed. In Isaiah's poem about the LORD's servant, the same verb says he carried pains that belonged to other people, and then that he will carry their guilt. English splits these into separate ideas, patient endurance in one case and legal liability in the other. Hebrew keeps one idea: a load on a back.
Two verses use the verb in unusual shapes whose exact force is debated: livestock described as weighed down, either loaded for work or heavy with young, and a grasshopper in a poem about growing old that either becomes a burden or drags itself along. Both keep the same picture of something bent under weight.