Different knot shape and possibly other fibres used for the SimFix. But yes, it feels small. On paper 25/52 (compared to 27/52 of the Chubby). That might actually be factoring in my feeling that I can’t build up a proper lather with it, being used to much bigger [badger] brushes, which can hold more.
Synthetics seem to be more sensitive to density and loft than their badger counterparts (which are bad enough!) Muehle synthetics also feel smaller than the knot size would suggest but they arent particularly dense which combined with a taller loft gives them good splay which increases their presence on the face... Same with Plissoft knots, although they swing too far the other way and usually end up some sort of springy mop.
I think Simpson got it just about right with the Chubby 2 synth, I know many out there disagree and found too much pressure was required to make it splay but treat it rough for a week or so and either the knot relaxes or the owner gets used to applying the extra pressure, either way it just feels right. Your description of the SimFix sounds similar to how I found the Classic 1 synth, I think it was Dipesh who described it as "like trying to lather with your elbow" - I cant improve on that description
