Hi Rossoneri,
Yes I would think they would stretch. I have quite a large tattoo between my shoulder blades of an old fashioned and stylised compass (i'm an Oceanographer so kind of makes sense!). Anyway, I had the same problem with this, when i first started diving etc. i only used to be of medium build, but over the years of carying the equipment, diving and working out etc, ive got alot broader accross the shoulders and this has distorted the picture a little. I went back to the tattooist (whos a good mate) and he suggested having the edges removed and then he could elongate it properly to make it fit and look better.
So this is what i did and the whole thing only cost around £200. The only thing is, when i get older and stop working out etc, if i lse my body frame i will have to do the whole lot again!
Hope this helps a little. Probably best to talk to your tattooist first though.
What are you planning to get done?