Below is a quote from a subreddit that says:
I haven't kept up on the research, but the last thing I read back in college looked at weight lifters and long distance runners. Both types had larger than normal hearts. The chambers of the weight lifters hearts had thick, walls, and were smaller. The runners had huge chambers but thinner walls. I do both, and my heart is larger than normal, but I've no idea what the internal structure looks like.
Other comments resemble the one above. Is that correct? Does that mean that cardio is better for lowering your heart-rate(e.g. from 70-80 to 50-60)? Also if the heart thickens the tissue and reduces the space in the chambers, doesn't that force it to beat faster?
P.S. this could be more appropriate for the fitness stack exchange but I wasn't sure where I'd get more accurate responses.