XTAL resonance increase the voltage
There is effectively an inductance, caused by the mechanical arrangement of the piezelectric crystal, called the "motional inductance". 

Here is an LTspice model of a 4MHz crystal using values for an ECS 4.000MHz crystal. 



I have simulated your probe with a 10M\$\Omega\$ resistor in parallel with 3pF. Crystal parameters from this page. 

The resulting Bode plot is here: 



The peak output is about +30.6dB at the resonance, which is even more than you are seeing.