If all cotton fibres were the same length, they would all be equal in strength. It takes many more short fibres spun together to make a yarn with the equivalent strength of one made up of longer staple fibres.
Hence the thickness and length of a fibre will help to determine the overall thickness of a spun yarn. Using a lot of shorter fibres will add bulk and the result will be a thicker yarn. A thicker yarn will limit the number of yarns that are able to be woven into one square inch, giving a lower thread count and a bulkier or heavier fabric. True high thread counts cannot be woven with short staple fibres.
Long staple fibres, like Giza 45, require less fibres to spin a yarn. Using less fibres will ensure that the yarn retains its strength but will be noticeably finer and smoother overall, as there are less ends protruding from the spun yarn. A super-fine yarn has the ability to be woven closer together and tighter than a thick yarn, allowing a very high thread count but with minimal bulk and superior strength.
Put simply, the longer the fibres, the finer the yarn that can be woven. This enables a higher thread count and a finer and smoother feel. A 1000 thread count is intended to be a very fine fabric. Despite misconceptions, a high thread count does not necessarily ensure the fabric will last longer or wash better rather it will produce a particular luxurious feel. Remember, it is not the thread count but rather the quality of the cotton and the hand of the fabric that is the true test of quality.