  The R package ecosystem is expanding fast and dependencies among packages in
the ecosystem are becoming more complex. In this study, we explored the package
dependencies from a new aspect. We applied a new metric named "dependency
heaviness" which measures the number of additional strong dependencies that a
package uniquely contributes to its child or downstream packages. It also
measures the total reduced dependencies in the ecosystem when the role of a
package is changed from a strong parent to a weak parent. We systematically
studied how the dependency heaviness spreads from parent to child packages, and
how it further spreads to remote downstream packages in the CRAN/Bioconductor
ecosystem. We extracted top packages and key paths that majorly transmit heavy
dependencies in the ecosystem. Additionally, the dependency heaviness analysis
on the ecosystem has been implemented as a web-based database that provides
comprehensive tools for querying dependencies of individual R packages.
