It's a big mistake to think that a particular thread is too small for the job in hand. Firstly, any form of upholstery thread will be immensely strong and will certainly be strong enough for anything we are likely to do; the strength of the final workpiece is determined much more by the quality of the stitch and these machines produce that par excellence. It's a really complicated subject if you go into it fully, but there's really no need, you'd be surprised how strong a seam you get even in leather and very heavy fabric just by using a good polyester "sew-all" type thread. Much more important is to get the tension right, so that the stitches link in the middle of the thickness of the material being sewn. Next time you get the chance, look at the thread used in a good quality handbag or outdoor garment - you'll be surprised at what you find.
If you feel the need to explore the subject further, you'll find stuff online; the art of it is not "bigger is stronger", but using the correct thread for the job. Hope this helps,..................atb mac