The phrase "significant contributor" means someone or something that has made a big and important impact or contribution to a certain situation or outcome. Full definition
A modest contributor in that past context for reasons that say it will be a more significant contributor in our current different context. (theinconvenientskeptic.com)
The countries that you mentioned are very, very significant contributors, and they're very deeply involved right across the program and I would very much want to encourage that to continue. (sciencemag.org)
During the next century, however, glacial melting will almost certainly become a much more significant contributor. (issues.org)