The word “stronger” means simply having more strength than, however when saying A is stronger than B, it does not imply that B is not strong just that A has more strength (B is strong but A is stronger). Therefore I am looking for the true antonym for this, in that I would like to say that B has less strength than A but is not necessarily weak. I don’t really like “weaker” as an antonym because it implies that the subject is weak, when they may not be.
Eg. Consider the difference between superman and batman; saying superman is stronger than batman seems fine, but saying batman is weaker than superman to me seems to imply that batman is weak, which is not the case since compared to an average person he is very strong, he just has much less strength than superman.
As you allude to, the terms stronger and weaker are strictly relative. Is Batman weak? Not in any kind of absolute sense. However, strictly compared to Superman, he is. This is true with any superlative. Is our sun small? Certainly not, when compared with any other object in our solar system. However, when compared with other stars that are 100 times larger, it’s minuscule.
If you don’t want to use the term “weaker”, you can simply say “less strong”, or otherwise clarify your position:
Batman is weaker than Superman, but still strong.
Batman is less strong than Superman.