Actually, I think this is just an old wives tale! Your sleeve is constructed from the non-stretchiest part of the stomach, with the fundus (the stretchy part) being discarded.
Read some of this researches I made about it and few things I found out. Yes, you can gain weight by over eating etc. but even if the pouch stretch it won't be a lot.
This one Is from another forum. And this person had three pregnancy after her surgery.
http://m.obesityhelp.com/forums/amos/4145194/It-is-VERY-VERY-VERY-hard-to-stretch-out-your-pouch/
ultravas02 reacted to leyvis for a blog entry, Doing a little research on how true can be that your pouch can stretch.