iMessage is not emailing through texting.
I think you have a misunderstanding of the way iMessage works. An email address for iMessage is simply an identifier, just like a phone number is. iMessage can use either or both to deliver a text message, but it's still a message and not email.
Someone who uses that email address can login to iCloud on a Mac, an iPad, an iPhone or any other iCloud capable device. iMessage thus knows where to deliver messages.
Messages sent using iMessage are not email, even though one or more users may use an email address as their identifier. They are text messages. But they are routed through Apple's servers and NOT your cell carrier. Because they are routed through Apple's servers, Apple can dicate how the structure works and thus can use an email address as an identifier. Because Apple's servers are being used for iMessaging there is virtually no limit to the size of photos, video, or any other attachments that you can send. There is also no limit to the amount of characters you can use. Lastly, because iMessage goes through Apple's servers you do not get charged for texting by your cell carrier.
Email uses email addresses of course, but email is not instant messaging or text messaging, nor is it an iMessage. An actual text message goes through your cell carrier and is subject to any limitations your cell carrier puts on text messages.
