Not only is the difference small it is not even always negative.The die itself only makes a small difference in total hit points and d8 does not indicate low hit points.
Considering the distribution 17% of 10th level Monks will have as many or MORE hps than a 10th level fighter with the same constitution. That is assuming you take max hps at 1st level, if you roll at 1st level too that number goes up to 20%.
So it is very possible for your d8 monk to have MORE hps than your d10 fighter.
Right now I am playing a level 8 Wizard, level 4 Rogue with a 13 constitution and 81 hps. I rolled 61hps on 8d6 and 3d8 or an average of 5.5 per roll and remember most of these were d6s
At the other extreme, I was DM for a guy who was a 8th level fighter 4th level assassin who had 35hps.