With iOS 9.2, it's easier to send large attachments using your iPhone. The Mail app now supports Mail Drop, a feature that was introduced on Macs with OS X Yosemite. Mail Drop lets you send attachments up to 5GB in size by using iCloud as a temporary way station.
As with OS X, you do not need to initiate sending an attachment via Mail Drop; the Mail app will offer it as an option when you attempt to email a large attachment. When you tap Send on an email with a large attachment, the Mail app will offer to use Mail Drop, telling you the attachment will be available on iCloud for 30 days for your recipient.
To test Mail Drop on iOS, I emailed a number of videos shot on my iPhone 6S. What I found was that if I was using the Mail app and attached a video, the Mail app would compress it and send it as an attachment. If I was in the Photos app, however, and selected a video to share and chose the Mail app as my means to share it, the video was attached without any compression.
Apple hasn't stated was the minimum file size is for an attachment to get the Mail Drop treatment, but my tests show it's 20MB. I emailed a 10-second video that was 20.7MB and the Mail app offered to use Mail Drop. When I emailed a 9-second video that was 18MB, the Mail Drop offer was not made, and the file was sent as a normal attachment.
Recipients of an email with an attachment sent via Mail Drop will receive a link to download the file from iCloud. The email will also state the date until which the file is available.