Engineer. Not a retcon.
This was always going to be a problem once resolved. Deal with it.
Engineer.
It's not the same character. If the Space Jockey and the Engineer seen in 'PROMETHEUS' were the same character, there would be an issue. As they are not, any argument about size is
completely disqualified. It's not the same spacecraft. The vehicle we see in 'ALIEN' is left out in the open, corroded and eroded. The pilot has been dead for anywhere between 10 months to 10,000 years. We don't know. The Juggernaut in 'PROMETHEUS' was a secure vehicle unlaunched and garaged in a hanger against the elements. Very different conditions apply here. Just leave your car outside in a car park for 10 years and compare it one that's been warehoused in a garage.
Besides, what's to stop the Engineers from upgrading their space faring technology? We've gone from steam to electric and diesel and trains still look much alike as they did when first invented. Ships... cars... tech changes but function defines form.
Finally, they engineered us, right:
So why can't the Engineers have extreme differences in sizes too..? Answer that one, Ding-dong*
-Windebieste.
*That is, anyone who has proof the Engineers
only come in one size and flavor.