There are many reasons why there is a size limit on the files..... the time it takes to render being the main one...
It's not like the days gone by with film - where the bigger the negative or tranny the better the print would be.
I'm sure most of you will have heard of "interpolation" which normally is talked about when making prints larger than the file would do at it's native resolution - however, it is also the same going the other way - and throwing away pixels to fit a smaller size does also effect the image.
As an example - I had a person send me 3Gb of a file - god knows how long it took for him to upload - must of been a few days - and it was for 200 6x4 prints! He has saved the files from a top of the range canon dslr at quality 12 - and they were huge. Not only did he waste a load of time uploading - it then log-jammed our internet connection for an hour and a half - then our render computer for an hour - then if the job had opened ok it would of been a couple of hours to render. When he re-did the job at the correct size and quality (10 in photoshop or 85% in Lightroom) the job uploaded in an hour - and printed in 10 minutes....
The size limit imposed is to stop this.
As most High Street printing is done in ASDA/TESCO/Boots you are left to the abilities of who is on to print at the time - and to the level of skill of whoever has set up an maintained the equipment. Now some will be excellent - and others very poor. I heard of one store that ran no balances on their printer for a couple of months whilst they had no manager - the prints varied in colour - the 6" were bright red- the 8" were bright blue...
A proper lab - by my standards anyway - is where people have the techincal skills to balance their equipment and to care what comes out looks "right" - and if it doesn't, know what to do to correct it. Now, many owner operated minilabs will do this - as will most prolabs.... but the 17 year old working for minimum wage in a place where no-one knows or cares.....
As someone said - owning a hammer and saw doesn't make you a carpenter - or owning a top of the range camera doesn't make you a photographer.... and working with a printer when you have no idea of how it works - how to sort it when it doesn't, and knowing the difference between a blue and a cyan print...
Back to the main issue - no-one needs a jpeg anywhere near as big as 6Mb for any size print that a minilab will produce. It needs no bigger than the physical size of the print at 300ppi saved at quality 10 (in photoshop) - no in any colourspace other than sRGB. Believe me - I spent a couple of weeks testing every possible setting when I bought my third minilab (which took my investment to just under £250,000 in 10 years).