Why is MAR high on the first of the month?

The reason you will see a larger increase in MAR/Credit consumption on the first of the Month is due to all rows being un-active again. Any row that is modified on the first of the month will become active and count towards MAR. The remainder of the month will usually be less as the same rows may be modified again but are already active and will not accumulate MAR again until the following month.

Why are there fewer Credits used as MAR increases?

The reason for the different amount of Credits you have been charged for the different amounts of MAR is due to Fivetran's Cost Per Credit Calculation which reduces the cost of MAR as your usage increases.

Our Service Consumption Table displays the ratio of MAR to Credits:

More info on this: https://fivetran.com/legal/service-consumption-table.

How MAR is translated to credits?

You can get an estimate of the cost for your expected MAR in credits using the Service Consumption Table (see above) along with the formula:

Credit Cost = Base Credits per Month + ((actual MAR - Monthly Active Row Threshold) * Incremental Credits per Million MAR)

Ignoring the incremental values for now, we can get a broad idea of the cost in credits by looking at the "Monthly Active Row Threshold" and the "Base Credits per Month" columns: 1 million MAR will cost 1,000 credits, 10 million MAR will cost 2,000 credits, 100 million MAR will cost 4,000 credits.

If you want to get a more exact figure you can use the formula. Here are some examples:

For 8 million MAR:
Credit Cost = 1000 +((8 - 1) * 111.11) = 1777.77

For 11 million MAR:
Credit Cost = 2000 +((11 - 10) * 22.22) = 2022.22