Ok so there may be some cause for concern here and it could very well be worth talking to your doctor.
Does this same sensation and burning feeling happen when you are not wearing nappies? if so it could be an indication of an infection and you should definitely consult a doctor.
If it is only while wearing nappies then there could be a large number of possibilities to why and also options to help prevent it.
I am not a medial professional so I can't really advise there but I do have a lot of experience both personal and caring for others in nappies so here are some potential possibilities.
First of how long are you wearing wet ones? if you are wearing for longer periods and going from one nappy to another the skin could be getting sore/damaged even with no visible rash so that even right as you are wetting it hurts.
Is there any redness/spots? this could indicate a nappy rash which can be treated and if recurring usually means you need to change more often and clean more thoroughly.
On that topic this may sound patronising but make sure you are properly cleaning and drying the area, many people don't think much about washing and just wipe around with a wet soapy flannel once or twice a day. you need to make sure all the soap is properly rinsed off as it can linger you also need to make sure (especially as you say you are a bigger person) that you get into all the little creases/crevasses and folds of skin.
Also generally for larger people you may need to wash or at least rinse/wet wipe around more often as any sweat or moisture can build up through the day and cause problems. Baby powder can also help drying and reduce moisture but make sure to only use the smallest possible amount needed to dry the area or that builds up and irritates too.
Do you shave down there? if shaving regularly you may be causing micro cuts/abrasions or razor burn especially for the area above the clit, these may not be visible or even cause any bleeding and urine could be causing pain from that, in this case changing the type of razor/creams etc. you use might help.
How well hydrated are you? knowing that the average person should drink around 12 to 16 cups or 2.7-3.7 litres per day and also over the course of the day not in large bulks all at once, this should probably be increased for larger or heavier people as more fluids are needed to maintain the body. It could simply be that you are not drinking enough or often enough so urine is becoming concentrated.
You may also want to explore the foods you are eating, while this will affect stool more than urine it does affect both, if you know your urine is acidic try to eat more foods that will balance that out a bit.
Second to that is if you are at all AB do you use a paci or suck your thumb/other objects a lot, pacis and thumb sucking cause the mouth to generate more saliva which like anything else has to go through the guts and is acidic by it's nature so reducing the amount you do this can reduce acidity a little.
Do you have drier or flaky skin? if so using a moisturiser may be of some help though maybe not by much.
As with contact dermatitis Skin problems can be caused by combinations of substances that separately you are fine with, i.e. you might be just fine using a certain hand soap and getting a cleaning fluid on your hands separately but if there is soap on you hands when the cleaner touches them it causes problems. In this case using a different soap or bodywash etc. to clean yourself down there might help as the one you have may be reacting to urine.
In the same lines some people (and especially so with babies) have problems using certain brands of nappies or wet wipes. some babies can't wear pampers without regular skin problems but are fine in a store brand for instance. Changing the brands you use and trying something different may help. If you can have reactions to toilet paper then it is quite possible that this may be a cause though you would probably be seeing it more on external areas of the skin than inside the labia for nappies, the wet wipes you use to clean yourself with would be reaching those areas.
A simple way to test if it's the wipes would be to buy "dry wipes" like
these, you can use them with just a little bit of water at first and then try with your preferred soap if it seems to help. They do not contain any chemicals, cleaning agents, soaps or scents that could be causing a problem.
Another potential solution or support option may be applying a good barrier cream (make sure it is a barrier cream not a rash healing cream) around the labia and other areas, this prevents urine contacting those parts and may help.
I would still recommend consulting a professional doctor if this persists though as it could be a symptom of something that requires other treatment.
