And the short-lived LEGO Quatro brick was...you guessed it...four times larger than a regular LEGO brick. Still compatible with both other sets.

https://en.brickimedia.org/wiki/QUATRO

I think these are biggest ones you can buy (or used to) not sure on the size comparison - at a guess x8 to x10

45003: Soft Starter Set https://www.rapidonline.com/45003-lego-soft-starter-set-70-1...

I had loads of fun playing with these in the lego centre (forget the kids!)

That softness is critical. Not for the kids, but for the adults who have to clean up after them.

Stepping barefoot onto a Lego brick hurts, but stepping barefoot onto a Duplo brick is much worse. Those things look innocent enough, but in the dark they turn into veritable caltrops!

You think so? The Lego bricks have sharper edges and also I think that you put the same weight (of your body) on a comparatively larger area on the Duplos, so less pain. But, well, who am I to argue about your experience. (Never stepped on either of these in our living room although we had both systems).

They have those soft ones at Legoland. They put them in the water park (they float!). I'm not sure they're compatible with regular bricks though.

They are probably partially compatible. With Duplo for example, it's easy enough to stack Duplo on top of regular LEGO bricks, but not the other way around. For stacking regular bricks on top of Duplo, you need to have bricks of the proper multiple in each dimension -- they need to be full height and a multiple of 2 in the other dimensions. The Quatro bricks are compatible in the same way -- you can easily stack them on top of Duplo or regular bricks, but not the other way around; You probably also need to do a transition layer from Quatro to Duplo to regular bricks.

I've seen people use Duplo and Quatro for space-filling when they needed a large amount of structural brick somewhere that won't be seen in the final model. Think having a LEGO city setup that has an underground level.

And the even shorter lived LEGO Octro brick was… you guessed it… 8 times larger than a regular LEGO brick.

Checkout the compatible “Marble Run” from Hubelino. Good build quality and loads of fun. Not affiliated.

https://www.hubelino.com/products/hubelino/marble-run/

As an aside, standard marbles are almost exactly 2 Lego studs wide, which makes it easy to build marble runs using just standard Lego pieces. For instance, you can build a marble lifting tower for the start of a marble run that uses a 2-stud by 2-stud hole in a 4x4 (or 6x6 for strength) tower.

3D printing similar parts is a fun option too: https://www.youtube.com/watch?v=Sb1c3VqqfTE

There is also Primo (first) for really little kids, pre verbal even) which is 2x Duplo and yes, can interlock with normal Lego bricks.

ETA: looks like it was unfortunately discontinued: https://en.wikipedia.org/wiki/Lego_Baby . This Lego was great for my kid to learn motor skills like orientation, insertion, and removal (along with things like wooden veggies that connected with velcro).

Obligatory quatro is twice bigger yet (and all are compatible together!)

Also Wikipedia mentions duplex and not duplus, but whatever

I mean the System in LEGO System in Play extends to the entire LEGO universe. Shouldn't be a surprise that they are compatible.