the term "big" is a relative term that guys use in comparison to what they see or experience or feel.
I think that many times we compare our own cocks to what we see in pictures or in porn, and this makes us all feel small or average at best. But when a bottom is getting fucked, he is comparing the feeling he is experiencing to other guys who have fucked him.
I always thought I was average, probably due to the first part of my comment above, but then sometimes hear guys tell me I am big when I am fucking them. At first, my thought, like you, was that they were just humoring me. But I've come to believe that in that moment, my cock feels big to him. I've also realized that I am just a little thicker than some, though I think pretty average in length, and that it is my thickness that they feel. So when guys say that, it first makes me feel good, but then I also take it as a cue to check in with him to ask if he needs me to slow down, add more lube or spit, or even stop if it really hurts. I try to be a respectful top.
So I suggest to accept the compliment, smile, and make sure the bottom is doing ok and enjoy it!