Oops, getting kind of back on track to what the original post was about.....
My experience from cycling is that it tends to make my running suffer (ok, not exactly back on track.
But the opposite way round, I do come from running!). My guesses as to why is that different muscles get looser and tighter from use than what happens in running (after hundreds of hours of cycling and basically no running I just did a short run today, my calves felt as tight as hell while I was running! Very weird feeling and nothing like what I'd had before as a runner), also I suspect I pick up a lot of bulk from cycling. Partly because cycling is more of a strength sport than running, partly because cyclists' bodies don't tend to be such a skinny build, and partly because various muscles that had been smaller start to be used more and gain mass. Plus there are sure to be other reasons.
I'd expect to a certain extent what I'm guessing is happening here would also apply to the reverse.