Estimate data used sharing files.
Sharing a file generates traffic once on the way up but once per recipient on the way down, and recipients frequently download more than once — re-fetching after a device change or losing the original. Total traffic therefore multiplies with the recipient count, which is why a single 25 MB file sent to thirty people is closer to a gigabyte of transfer than to 25 megabytes.
Share-once, download-many
Upload MB = files x size; download MB = files x size x recipients x downloads per recipient; total GB = (upload + download) / 1024
Only for peer-to-peer or direct transfer. With a cloud service the upload leaves your connection once and the downloads come from the provider's servers, so the downstream figure lands on recipients rather than on you.
Because recipients rarely download exactly once. Opening a shared document on a phone and then a laptop, or re-fetching after clearing a cache, means measured downloads per recipient typically run 1.2-2.0 in practice.
Share a link to a single stored copy rather than attaching the file, and compress or downscale before sending. A 25 MB presentation with uncompressed images often drops below 5 MB with no visible quality loss.