When it comes to wrestle it all depends of how talented and how trained a superstar is so yes,someone whose not so tall can win against a taller superstar but only if he is really talented and trained for years as his oponent - here's a example: Both Seth Rollins and Roman Reigns debuted together before they split and start a solo cariers,both trained for years and right now they are once agin in a rivalry but Seth Rollins is the one who wins almost all the times vs Roman especially when it comes to fight for a title and Seth is not so taller as Roman but he is more talented while Roman may be taller and more buffed but he is not so talented as Seth,not so fast and Seth can easilly play mind games on him too which is another way to secure your win! Now here Axel seems buffed and probably hit the gym for years while dating Lena before they broke up while Ian only started hitting the gym for few months,after meeting Lena and Jeremy conviced him to train together,or he may not train at all (if you actually decide to make Ian not so strong) or train only for keeping his body in good health but not trying to train professional - so in this way no matter if Ian is less taller then Axel he will not be able to win against him as Seth do against Roman because Ian is not so talented and only start training recently while seeing how buffed Axel already is it means only one thing - he trained for years,hell even Jeremy didn't want to fight him back at the bar where he,Ian,Ivy and Jess meet Axel...
PS: The only way for Ian to win against Axel is by trying playing mind games on him but is not sure if Axel is easy to play mind games on him.