"It's important to note that the amount of pain is not necessarily related to the extent or size of the growths. Tiny growths, called petechiae, have been found to be more active in producing prostaglandins, which may explain the significant symptoms that seem to occur with smaller growths." http://www.crat.org/Endometriosis_Info.html
"There was significantly different distribution of nerve fibers in multiple endometriosis lesions, which was correlated with dysmenorrhea, anus pain, dyspareunia and chronic pelvic pain, not with clinical staging." http://www.ncbi.nlm.nih.gov/pubmed/20646536
"The presence of endometriosis-associated nerve fibers appear to be related to both the pain experienced by women with endometriosis and the concentration of PF cytokines; however, this association varies with the lesion location."
http://www.ncbi.nlm.nih.gov/pubmed/22154765
"There was no difference in the density of nerve fibers across the menstrual cycle in peritoneal endometriotic lesions. These findings may explain why patients with peritoneal endometriosis often have painful symptoms throughout the menstrual cycle." http://www.ncbi.nlm.nih.gov/pubmed/21334610
"Initial work on mapping of pain associated with the endometriosis lesions resulted in some thought-provoking findings. The classic black lesions were found to be painful in only 11% of patients when the lesion was touched. Similarly, white lesions were painful in 20% of patients with red lesions at 37%, and clear lesions at 32% were the most painful (Table 1). These results added further reason as to why initial therapy had such poor results. Surgeons would only “see” the black lesions and removed them, but these were the least painful lesions. The most painful clear lesions were not “seen” at laparotomy and therefore remained, as did the pain. What became apparent next, while mapping the patient, was the fact that the pain extended 28 mm beyond the visible border of the lesion onto what looked like “normal” peritoneum ((Figure 1). Therefore, if the surgeon only removed the lesion at its border, the microscopic disease in the previously identified normal looking peritoneum was left, and persistence or recurrence of the symptoms was encountered." http://www.ncbi.nlm.nih.gov/pmc/articles/PMC3015350/
"Women with endometriosis and pelvic pain almost always have fine, unmyelinated nerve fibers present in the functional layer of endometrium, and these nerve fibers are also greatly increased in the myometrium. Women without endometriosis almost never have these nerve fibers. These nerve fibers may also play a role in pain generation." http://www.livingwithendometriosis.org/2010/03/28/nerve-clusters-without-myelin-sheath-found-as-culprit-in-endometriosis/
"Hormones and chemicals released by endometriosis tissue also may irritate nearby tissue and cause the release of other chemicals known to cause pain....Some endometriosis lesions have nerves in them, tying the patches directly into the central nervous system. These nerves may be more sensitive to pain-causing chemicals released in the lesions and surrounding areas. Over time, they may be more easily activated by the chemicals than normal nerve cells are. Patches of endometriosis might also press against nearby nerve cells to cause pain." https://www.nichd.nih.gov/health/topics/endometri/conditioninfo/Pages/symptoms.aspx
"Our group discovered that ectopic growths harvested from ENDO rats and women with established endometriosis develop their own C-fiber (sensory afferent) and sympathetic (autonomic efferent) nerve supply. The supply is derived from nerve fibers innervating nearby territories that sprout branches into the growths [10], [11]. This discovery suggests that, rather than the growths alone, it is the ectopic growth's own innervation that is a major contributor to the maintenance and modulation of pain in established endometriosis." http://www.plosone.org/article/info%3Adoi%2F10.1371%2Fjournal.pone.0031758
So pain is influenced by several factors- the location of the endo, the "type" or "stage" (clear, black, red, etc) of lesions, how much innervation is there, how much inflammatory chemicals are being released, if adhesions are pulling on anatomy, if other conditions such as pelvic floor dysfunction, interstitial cystitis, or adenomyosis is present. Then add in other things that influence how any type of pain is felt and you can see there are many factors that come into play.
