Leather is dead flesh, but like living flesh, it has some give to it—so, you can stretch a tight leather item. Glove leathers (such as kidskin) are softer than cowhide shoe leather and are easier to stretch. In Victorian times, tight gloves were fashionable, and most households owned an ivory finger stretcher, which looked like two ribs on a hinge. Such tools are scarce today, but you can likely get by with simple water and isopropyl alcohol. Try the easiest ways of stretching leather first, then more aggressive methods as needed.
Leather stretches naturally with wear. Wear the gloves for a few days--constantly. Between your body heat and normal use, you likely will find that the gloves stretch naturally to fit your hands.
Take care to lace your fingers and squeeze tightly; this will stretch the fingers for length. Leather has a directional grain. A good pair of gloves will be made with the grain running in the direction of the fingers, and so will stretch in length.
Water is not the absolute enemy of leather; it helps it to stretch and is part of the tanning process.
Mix a solution of three parts water to one part isopropyl (rubbing) alcohol in a spray bottle. The alcohol will help the water dry more quickly. Spray the gloves, inside and out, enough to dampen them well, and wear the gloves until they dry, which might take two to three hours. Repeat as needed.
If the above steps fail, apply a shoe stretching spray. This spray softens and breaks down the collagen in the leather, the fibrous structure that gives living flesh its firmness and leather its toughness.
Spray the gloves inside and out, and wear the gloves until the spray has dried. The chemicals are dangerous to ingest but not dangerous to you on your hands or feet.
Treat the gloves with a lanolin-based conditioner. Leather shrinks as it dries, and keeping it conditioned will prevent shrinkage. Steer clear of oil-based conditioners, like mink oil or neatsfoot oil; these will make the gloves tacky, and stain your clothing.
As the author of the "Leather and Latex Care" manual put it, "If you can't fit into the the item to begin with, you probably won't get the results you want." If the gloves are a bit tight, you can stretch them to fit. You might gain perhaps 5 percent in length or width, but you cannot go up a full size--from small to medium or large to extra large.