A standard crash pad weighs 6-8 kg (13-18 lbs). On a long approach with multiple pads, the weight adds up fast — three full-size pads is 20+ kg before you add the rest of your gear. Lightweight pads cut that to about 4-5 kg per pad, making bigger pad stacks practical and shorter approaches less brutal.
The trade-off: lightweight pads use thinner foam, smaller dimensions, or both. They're great for protecting low-angle problems and lower falls, less great for highballs where you need maximum cushioning. The four pads below balance weight against protection differently — pick based on the problems you actually climb.