temp is fine, in fact ~25C is a good target if you want to control it tightly. This happens to be 77F. This is actually where you want to be if trying to max out rate of photosynthesis, but this takes a lot of environmental control to make it worth it and then might as well boost atmospheric co2, too... complicated/more expensive.. not sure what you r goal is...
70-75% is too high, i'd set fan speed to mitigate that, but don't drop to 45% if maintaining 77F.
RH above 60 can be a bit dangerous, so definitely allow your fan to reduce it in any loosely controlled environment contexxt. If not going all out on sophistication a couple degrees lower with slightly lower RH% might be better than 77F/60-65%.. less risk of microbial growth, maybe slightly less optimal relative to yield, but in a loosely controlled environment, no point in being too picky as potential gains won't be fully realized. (i am in same boat fwiw.. i loosely control environment... i avoid extremes and let it do its thing)
77f, 60-65RH% results in a good vpd of 1.27.. this should be safe for any mature plant and maybe a bit high for a seedling. Even 55% RH would be okay for a mature plant, though you may need to reduce concentration of feed a bit with an elevated transpiration rate in plant the further RH drops at those temps -- rate of transpiration is primary driver of water intake in roots.
genetic variety accounts for a huge difference in plant outcomes, and more so in autos due to how they are created vs photoperiod plants. they are inherently more volatile. so, you are more likely to see more drastic differences in resulting sizes.
Is it stunted? how are the internode lengths? this is the length of stem between 2 growth nodes on any limb. Are they super tight? are they pretty much like the plants that are growing larger?
if they are tight, it's possible it's gettign too much light... eithe rmove to a corner or raise the light above it slightly if it doesn't negatively impact other plants. you want to maximize your best plants... runts can go in the coner.
but, see how it reacts if that is the case... should see it start to grow out in 7-10 days, if it was stunted and caused by too much light.
if nodes are seemingly similar to others, i'd just chalk it up to a smaller plant.
Check out a vpd chart... figure most recommend 1.1-1.5 for a mature plant... some sites sugest different things.. tells us while this is an important factor it is not well researched yet as far as what is truly best. it is indiscreet... multiple combinations of temps and rh result in same VPD, but they will be slightly different results on plant too.