The only way to get this to look good is to install the taller/wider baseboards in the room with the skimpy ones.

Make all your baseboards the same. Don’t try to transition two different ones.

Ignore it. No one is going to look at your baseboards anyway.

Rooms that are open to each other should have the same baseboard trim. In fact it should extend into adjacent hallways. People pay attention to details.

Millworkman's idea for a plinth or corner block would work. Another option would be to extend the base on the left, miter the end and add a return that would die onto the base on the right. Or you could return the base on the left back into the taller piece it's installed over.

https://www.carpentry-tips-and-tricks.com/end-skirting-boards.html

This is what my kitchen installer did. Maybe it will be helpful in your case. The baseboard is in the hallway. The side of the kitchen cabinet (shown on the right side of this image) is fully covered by a panel. It would have looked awkward adding baseboard to this one small section of the kitchen, so he put this corner trim along the edge of the wall and a small flat trim (not quarter round) at the floor.

That ^^ might have worked if the corner trim was thicker than the baseboard. Thinner does not work at all.

I think all baseborads shoud be the same and which one is the one in the rest of the house? The treatment like Dina posted would maybe work.

You appear to have a wainscot in one room and not in the other. That's fine to transition to different baseboards. Since your wainscot base is 2-piece, I would do 2, maybe 3 miters like this:

In BLUE miter the low base around the 135d corner to die into the wider, taller, simple piece. Then in GREEN miter the chair-rail style piece to have a finished end. The RED simple piece can be either straight cut or mitered.

You may be able to put a white 1x3 or 1x4 board floor to ceiling on the gray wall corner, then butt the baseboard up to it, or even change out the baseboard to be a taller 1x board. If you have crown moulding this may not work. If your wainscoting stops mid wall, you might have to add something to the top of it. In other words, consider trimming out the corner floor to ceiling to create a ”column” effect.

If you have crown moulding, you could trim out the top corner of the new ”column” to meet or exceed the depth of the crown moulding.

If you can match the decorative baseboard, that would be better, I think.

Not ideal but this is what we did to transition from the stairs to the downstairs molding… excuse the dust and much needed touch ups. Nothing stays new very long…

3onthetree, you are correct there is wainscoting on the one wall. It is actually the entryway. Your diagram is so helpful, thank you!!!

Kelli_ga, this is another good option! I can match the decorative baseboards, that’s not a problem. Thanks so much for your help!!!!