Nylon strings for Classicals come specified in set tensions not gauges. In general, low/light, medium/normal and high/hard, and even extra hard. e.g. EJ44. Which did you fit? Medium aka normal is default fitment on Yamaha's C40/70/80 series.

If lightening your touch isn't working, try Pro Arte EJ46 hard or even EJ44 extra hard. Higher tension will also result in more pronounced tone as a rule.

It's normal for a fresh set of nylons to go out of tune spasmodically after a change until played a while.