Electric Circuit

I would expect parallel circuit giving brighter light.

Because the voltage across each bulb would be higher - source voltage.

For series circuits, the voltage will be shared by the series connected bulbs.

For accurate answers, we need to consider the capacity of the batteries, their internal resistance, the capacity of the light bulbs, their resistance and the numbers used.

Series circuit.

It is because the sum of each branch of the parallel circuit equals to the battery,

but one brach of the series circuit already equal to the battery