Hi Michael,

> Does anybody has an idea how to get this smooth transition
> between the legs and the belly while keeping the "hard edge"
> between the legs?

I think what you want there is to apply a fillet for the upper belly part, but no fillet between the 2 legs.

Something like this:

Started by drawing a profile curve:



Then select it and run Construct / Revolve to build one leg piece:



Select that leg and run Transform/Mirror to duplicate it on the other side, then select both legs and run Construct / Boolean / Union to fuse them together.

Then I drew another similar but larger profile above:



Revolve that one, Mirror it, then make another larger version in the middle:



Now boolean union those upper 3 pieces together - this will create edges where the pieces intersect each other, you can see the black edges appear here:



Now select that piece and run Construct / Fillet to round off those edges:



So I think the thing you're looking for is just a boolean union with no filleting for the lower leg portion, but then boolean union plus filleting for the upper part.

Hope this helps!

- Michael