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When there is an opposite voltage swing in two wires the coupling capacitance between them is doubled and this phonomenon leading to more capacitance is called miller effect which produces devastating effects in integrated circuits.
Miller theory states that :When you have 2 floating nodes with an impedance between them, this impedance can be replaced by 2 impedances to the ground, one at each node .Where Z at node 1=Zoriginal/(1-Av) and Z at node 2=Zoriginal/(1-1/Av) where Av=Vnode1/Vnode2.These formulae are valid for floating nodes and -ve Av or Av<1 .
A special case for capacitors and Av<<-1 (|Av|>>1)e.g common source amplifier, this gate-drain capacitance (C) can be replaced by a cap at i/p of value ≈ |Av|*C which is much larger than the original capacitor which really degrades the frequency response of your amplifier .Note also that Miller's theory can't predict added zeros by this capacitor .
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