What counts as a mass message credit?

In general, one mass message credit equals a single message to a single recipient or number. The character limit on a mass message is 320 which is technically double the size of a typical text message.

If your message exceeds 320 characters you would need to break it up into multiple messages.

Why can I send significantly longer messages in iMessage?

It's important to recognize that when sending messages on iMessage to another iMessage user, these messages are not sent via SMS. They are sent via Apple's messaging service. That's why messages can be much longer. If you send via iMessage to an Android device or other mobile phone, iMessage will convert the message to as many 160-character SMS messages as are required.