Feeble Funzel, disappointing

In my Maglite 3 D-size batteries for the LED module produces a much dimmer light than in the also compared with Maglite 2 D-size batteries. Either something is wrong here or module requires at least 4 and more 1.5V mono cells.
I have tested it on several occasions, even bought new mono batteries, nothing helped.

The LED module for 2 battery version looks much brighter than this one.

The module goes back unfortunately. Luckily I had cut open the package carefully and clean ...

Warning: Many reviewers write here of D-Cells. In Germany you do not understand the times, since the batteries be called something, namely mono (D), Baby (C), Mignon (AA), Micro (AAA), Lady, duplex and block. So much for the well-intentioned enlightenment