Some info about sewing machine needles:
http://www.joann.com/static/project/0704/PA00223.pdf
On sewing machine needles, the larger the number, the thicker the shaft of the needle.
For piecing, I like to use a 75/11 or 80/12.
Many people use at least a 100/16 for sewing heavy denim.
The numbers may be reversed - 75/11 is the same thing as 11/75.
Also - make sure that you are using the correct needle for your sewing machine and that it is installed properly.
Hand sewing needles are numbered the other way - the larger the number, the thinner the needle. A smaller sized needle in a given style is given a larger number. Example: A #12 between will be thinner and shorter than a #9 between.
It seems like a lot of demonstrations are done on that stiff, coarsely woven material with thick (14/90 or thicker) needles.
It can get confusing.