Tartan was originally woven in 27inch widths (long before the French Revolutionaries had come with the metric system), and some spcialist tartans are still produced on the old hand-looms. The Highland regiments originally supplied the men with lengths that varied according different factors, and was measured in "ells" - a standard public-use ell is still in place in Dunkeld market-place (an ell is about 45inches), where such cloth was sold until fairly recent times. The practice until the early 19th century was for the cloth to be cut in half and joined along its length, thus giving a piece of material half as long but double width, which was then pleated and formed into the belted-plaid as worn at the time.
Essentially, there are two types of kilt: the breacan-feile or belted plaid which was usully made of 12 ells of tartan, and the feileadh-beag, or little kilt (as we know the kilt today) and was usually made up of six ells of single width tartan - or about seven and a half yards of cloth. The seemingly long lengths of cloth are needed to both provide sufficient for the pleats, and to allow the pleats to be formed in such a way as to continue the sett of the tartan - a very neat skill.
Early single-width kilts were the full 27inches and worn high up the body - the standard modern kilt length is now 24inches. Woollen mills still refer to their 54inch (or the modern metric equivalent) as double widths.
A properly made kilt will have pleats of two to three inches deep (six inches of material is needed for a two-inch pleat) and will be as much as 11 layers thick in places. I have a 1932 army kilt, a 1958 hand-loom woven kilt and a modern bespoke kilt cut from a double-width. The army kilt has the least mterial and is made up of eight yards and 26 inches and the modern kilt is just more than nine yards in length (on a 32inch waist), taking into acount the turned-in ends which give the flat front portion the charcteristic frayed edge.
Anyone who doubts this should take a tape-measure to a Highland outfitters and check for themselves.