Kennyroy,
capio777 is leading you astray with all that nonsense about Centre of Pressure.
BMatthews is on the ball. You need to work out your Tail Volume Ratio.

Tail Volume Ratio (V-bar) is (tail area, divided by wing area) times (tail moment arm, divided by wing mean chord)

You are working out the mean chord (MAC) correctly by the looks of it, now measure the tail arm, which is the distance between the quarter chord point of the wing MAC and the quarter chord point of the tail.

That 108 in^2 tail is 20% of the small wing but only 13.4% of the big wing !!!!!
Your tail arm is short, let us say it is 1.5 times the MAC of the big wing, that gives a Tail Volume of 0.20 x 1.5 = 0.3. That is small, but not too bad. I would say that your Neutral Point (where to put the CG for zero stability) will be 35% to 40% of wing MAC. You like it that way I take it.

Next model. Same tail. Now 13.4% of wing area, and I'll bet the tail arm is now only 1.3 or 1.4 times MAC (because the MAC is bigger)
You tail Volume is reduced to 1.4 x 0.134 = 0.188
That is too small. Build a bigger tail or a longer tail arm or both!!!

My guess is that your Neutral Point has gone forward to 31% MAC so with a CG at 35% you are definitely unstable. A more forward CG should get your model stable, but I'd still suggest a bigger tail volume.

Stability is purely a matter of CG position. Anything can be made stable with a suitable CG. The sections have very little to do with it. I haven't seen a section with an Aerodynamic Centre further than 2% from the 25% (quarter chord) we all assume.
Forget Centres of Pressure. Have you heard a serious Aerodynamicist use them since the 1920s? Ignore the CP and anyone who uses it.
Believe the guys who talk about Aerodynamic Centre (or quarter chord point), tail volume, Neutral Point, Static Margin or Stability Margin (the distance of the CG ahead of the NP).