Maxi-size mega blocks are larger then duplo. As far as I can tell they have 2x the stud pitch and much larger studs.
What if-any compatibility is there between the maxi-sized mega blocks and either Lego elements or the smaller-sizes of Mega blocks.
Yes, maxi sized mega bloks are compatible with LEGO duplo and LEGO system pieces, but only if the duplo piece is 2x2 studs or larger or the LEGO piece is 4x4 or larger. Also, you can only connect LEGO system pieces and duplo pieces to the undersides of maxi size mega bloks.
The other brands generally fit LEGO bricks. What you have and the Duplo bricks work together. I noticed with any brand that isn't LEGO the pieces are compatible, but they are slightly off like they had a problem perfecting the proportions with the Lego brick. They kind of stick sometimes, and sometimes they are loose enough to just fall off.