We are more or less the same (I am slightly taller).
Couple of "problems" going too small. Assuming you are placing your straps in the widest position going short means the board will flex a lot less. This is in my opinion underestimated by riders that are not using the widest available stands. As for too narrow you get into the issue Herman is talking about - matching the width of the board up with your foot size for edging.
I got a 140x43 board. Went out in very choppy conditions the other day and even if my edging technique is hardly stellar I can't say I struggled much to keep it. Might have been an issue if I had tried to jump (wind wasn't strong enough).
To me this obsession with smaller boards reminds me a bit about the obsession with big gears in cycling. In certain circumstances for very skilled riders it makes sense, but for most people I'm not sure if it really does. If you are struggling to keep an edge trying to jump in challenging conditions I get it. Slightly bigger kite with a smaller board it makes sense. How many of those do I see on a regular beach on a regular day? 1? 0? I can imagine that among the posters on here there are quite a few very skilled riders - as they are interested enough in the sport to be on a message board, but I do believe that also sometimes gives the wrong picture of what is useful for the "normal" rider.
I can't give very specific advice as to which board it makes sense to complement your current. I wouldn't have the knowledge to do so. 138x41.5 is however very small for riders our size. Not so that you can't ride it of course (maybe it even makes sense to have such a jump between the boards to really notice the difference), but really not comparable to someone being 75-80kg and 20cm shorter. Personally I think you would have noticed a fairly significant difference between my 140x43 and the one you got (without knowing your board specifically). My previous board was 142x42 and even if I can't tell it is because of the size or the different design my new and my old are quite different. The old 142x42 was a lot more difficult to ride in choppy conditions.
Also worth pointing out I got a friend in the same weight range as me and he prefers smaller boards. Not sure if that is because they are better to ride or that old stigma of having big ones. In any case he is a lot more capable than me at handling small boards in marginal wind than me.
Now more confused than ever