Obviously, we're talking to a relatively new reloader here (farmerbuck), so let's get back to basics, so that he can learn. A case does not stretch during resizing. When a cartridge is fired, it expands to fit the chamber. If there wasn't room for it to expand in the chamber, then you wouldn't be able to get it in the chamber. In otherwords, the case must be smaller than the chamber in order for this whole shooting thing to take place. Generally, SAAMI specifications for chamber size are significantly larger than SAAMI specifications for case size. After firing, the case reduces in size to again be smaller than the chamber (otherwise, you couldn't get it out of the chamber). But, in virtually every situation, the resultant fired case is larger in diameter after firing than it was before firing, being on the upper end of SAAMI specifications for case size (assuming that your chamber falls somewhere in the specification of SAAMI, which in fact are quite loose). So now we go to the step of full length sizing, which is going to reduce the diameter back to something on the low side of SAAMI diameter specifications, i.e. case specifications, in order to assure that it will fit in any SAAMI chamber. We can't be "hoping" that the intended chamber is large enough and the case small enough; we must be certain). When that case is squeezed down in diameter, the material comprising the case (brass) must go somewhere. In the confines of the die, it can only go one place, and that is up towards the opening, i.e. the neck. There is no place else it can go! The obvious result is that the case is now longer. Basic when you think about it. The reason (at least a major reason) that some folks don't full length size, only neck size, is to eliminate this squeezing back down in size (diameter) and the resultant "growth" in length. Neck sizing works because you are putting the reloaded shell back in the same chamber (KEY--neck sizing is ONLY used when the same shooter is being employed) that it came out of, hence it fits. Even then, after about 4-6 reloads, the expand/shrink of shooting becomes reduced enough that a dose of the full length die is needed in order to get the case into the chamber. So, the reloader guy needs to realize that every time he full length sizes a case, it IS going to get longer. Depending on your individual chamber size and die size, this degree of lengthening may require trimming every reloading cycle, or maybeso only every 2nd or 3rd. If one chooses to neck size, trimming is generally reduced to that one in 5 or 6 reloadings that reqires full length sizing, because you are not squeezing that case down each time. Generally, it is quite advantagous to anneal at this point, also.
Basic point to all this yack is that yes, your case will increase in length significantly every time you full length size it (but you aren't "stretching" it), so what you are seeing is normal. If you full length size (forget the caliber/cartridge), you will have to trim on a regular basis, i.e. every or every second reloading.