Many manufacturers are now switching to the use of elasticated belts for use in their appliances. They have the advantage of offering a consistent fit and tension over a wider range of fittings. i.e. they have a better length tolerance than normal belts.
If you buy a new belt for a washing machine or tumble dryer it may be replaced by an elasticated version. You can normally tell because the size description printed on the belt includes the letters "EL" as in: 1196 J5 EL or 1196 J5 MAEL.
Sometimes the belts can be a very tight fit; even to the point where it seems impossible to get them on at all. They can simply seem to be too short even though they are meant to be the correct size.
The answer to this is to soften the belt before fitting. This can be done simply by placing it in (quite) hot water for a minute or two. When it becomes softer and more pliable, you then fit it to the machine. There will now be enough stretchiness in it for it to expand as its wound around the pulley (for example).
It will tighten to the correct tension as it cools down.