The weight matter comes up repetatedly and has popped up again on:
http://www.concertina.net/forums/index.php?showtopic=10430

This concern certainly is understandable some way since a (too) quick thought naturally will make the question motivated and of course it can not be denied that at extremes the weight has got importance for playing facilities BUT within the ranges when people mostly are comparing the instrument weights the significance thereof is neglictable.
Since new readers may have popped up as well I can't control myself from repeating some answers again ...:-)

1)The absolutely dominating work related to *playing* concertinas is the pumping effort.It may vary between 5-10kg at energetic playing
The major difference in playing effort between large and small concertinas is related to the end size - neither the weight, nor the length of the bellows but a long bellows needs more stabilizing work.

2)Carrying the instrument is a matter of usually 1-2kg and the variations people compare often concern for anglos say 100g more or less and a fraction of 10-15% of the total weight - just about the
detectable difference, and hardly ever an important difference regarding endurance of playing, not even standing

3) Pumping effort can not be avoided - it is essential for the musicmaking as well as the fingering on the keys.

4) Carrying effort on the other hand can almost entirely be eliminated if being an important factor. The instrument may rest on knee(s) when seated and hung up on straps when playing standing

5)Using a "neckstrap" however is NOT a recommended, particularly not the traditional thin cord.A broad( 50-60mm) strap is better, and the very best is using one or two broad elastic shoulder straps and very important: when hanging the instrument up in any kind of strap the playing position must be low(!) - elbow angle about 120 degrees or more - to achieve reasonably good effect from the arrangement. The traditional position with forearms horisontal when playing standing with a neckstrap can only be regarded as contraproductive or even absurd.