It's MMS. Generally, MMS is going to compress images to a tiny size after you've sent it to meet the maximum of the given server.
Unfortunately, MMS technology is eons behind the capabilities of even your basic cell phone now. I know it's easiest to send pictures and videos through MMS, but if the recipient is on a smartphone or even a messaging phone you should really use an alternate method such as email or an online sharing service.
Sometimes MMS will allow a decent quality picture through, but generally, that's the exception, not the norm. I wouldn't be surprised if phones started coming with MMS disabled by default. Most MMS messages I send don't even get through; I consider it lucky if the recipient receives it even though it's all compressed to hell.