A. Originally, a character's backpack could hold 400 stoneweight (whether the character could carry more or less is immaterial - only 400 would fit into the pack). However, with the release of Mondain's Legacy, this was increased to 550 stoneweight. Most persons reading this probably have Mondain's Legacy.
B. In addition, the overall carry weight of characters (pack+equipment) has a "safety margin" where one can still recall, while overweight; it is also the point that you do not lose stamina when walking. This is 1% of their total carrying capacity from strength, rounded down (but carrying capacities that are exact multiples of 100 count as the lower category). Typically, a character with 390 carrying capacity (a 100 STR Elf or pre-ML Human) has a safety margin of about 3 stone.
Strength Recall Safety Margin Test results from Test Center (using human test subject):
STR 28 (198 stone weight): Safety margin of 1 (could recall at 199, but not 200)
STR 29 (201 stone weight) Safety Margin of 2 (could recall at 203, but not 204)
STR 57 (299 stone weight) Safety Margin of 2 (could recall at 301, but not 302)
STR 58 (303 stone weight) Safety Margin of 3 (could recall at 306, but not 307)
STR 114 (499 stone weight) Safety Margin of 4 (could recall at 503, but not 504)
STR 115 (502 stone weight) Safety Margin of 5 (could recall at 507, but not 406)
STR 143 (600 stone weight) Safety Margin of 5 (could recall at 605, but not 606)
STR 144 (604 stone weight) Safety Margin of 6 (could recall at 610, but not 611)
Testing would then seem to indicate that the safety margin is
100 or less: None
101-200: 1 stone
201-300: 2 stone
301-400: 3 stone
401-500: 4 stone
501-600: 5 stone
601+: 6 stone (reachable only by STR raised by equipment/magic to the 140s)
C. As one has to add one's pack contents to the equipment worn, for total weight, you get the following examples.
*A human miner with 125 STR has a carrying capacity of 550 for his pack, but only 537 for STR. If his worn equipment weighs 77 stone, he can carry up to 465 (460 + the 5 stone safety margin for is total capacity) without being encumbered. However, he can weigh as much as 627 (the worn equipment + 550, the backpack capacity). If he is out mining, in a safe spot (and no danger of ore elementals from a gargoyle pick), he can keep going until he gets to 603 without a chance of losing ore on the next swing, 615 if willing to risk losing a double-ore from being human.
*The same miner, with only 20 stone worn equipment, would run out of pack space at 570 stone total weight. So, the miner can watch for being overloaded, and that would be a good indicator to stop and unload/smelt, as they can hold only 2 more large ore after going over the safetly margin.
*Now, take that character, and give them 144 STR from equipment. They now have a safety margin of 6, and a total capacity of 604 stone (610 with safety margin). With the 77 stone suit, they could mine up till they got over the safety margin, but with only room for 1 full ore over the margin. BUT, using the 20 stone suit from the second example, they would actually hit their Backpack's maximum capacity at 570 stone. They will never be overweight, but they won't be able to fully utilize their strength (but, this could, after all, come in handy if a red tries to curse them in Fel).
D. If one mines in the PvM facets only, in reasonably safe areas, this is the perfect excuse for a high-strength (110+) miner to use metal armor (nothing screams sellout like a non-tailor miner/smith wearing all leathers

). It can be a good visual aid for high-strength miners, to limit the chance of ore loss, to match your worn equipment (including weapon & shield) to the amount the character's total weight capacity goes over 550 stone, so you'll always know that when you hit an overweight (but not necessarily encumbered) state, you will lose large ore if you keep mining.
E. A person, assuming they have a rune book and a few other things on board, can carry at most 45 large ore or ~275 small (only tool being a pickaxe in hand), more often than not, closer to 40 large/250 small, depending on their supplies in pack. However, with the introduction of the fire beetle in Samurai Empire, few carry around ore anymore, converting to ingots as needed.