Why are my files larger on Proton Mail than on my device?

When attaching(new window) files to emails, you may have noticed that they appear larger in your email than on your device. You are correct. The files size increases due to the encoding and encryption(new window) process that is necessary for your file to be sent securely on the Proton Mail system. Due to this process, the file will increase by about 1.4 times its original file size.