Bigger number sounds better for the ISP.

The real answer?

Data is transmitted in packets. Each packet has a packet header, and a packet payload. The total data transmitted is the header + payload.

If you're transmitting smaller packet sizes, it means your header is a larger percentage of the total packet size.

Measuring in megabits is the ISP telling you "look, your connection is good for X amount of data. How you choose to use that data is up to you. If you want more of it going to your packet headers instead of your payload, fine." A bit is a bit is a bit to your ISP.

@Moneo @SigHunter Networking came to be when there were lots of different implementations of a 'byte'. The PDP-10 was prevalent at the time the internet was being developed for example, which supported variable byte lengths of up to 36-bits per byte.

Network protocols had to support every device regardless of its byte size, so protocol specifications settled on bits as the lowest common unit size, while referring to 8-bit fields as 'octets' before 8-bit became the de facto standard byte length.