Coherent superposition of electronic states, created by ionizing a molecule, can initiate ultrafast dynamics of the electron density. Correlation between nuclear and electron motions, however, typically dissipates the electronic coherence in only a few femtoseconds, especially in larger and more flexible molecules. We, therefore, use ab initio semiclassical dynamics to study decoherence in a sequence of analogous organic molecules of increasing size and find, surprisingly, that extending the carbon skeleton in propynal analogs slows down decoherence and prolongs charge migration. To elucidate this observation, we decompose the overall decoherence into contributions from individual vibrational modes and show that: 1) The initial decay of electronic coherence is caused by high- and intermediate-frequency vibrations via momentum separation of nuclear wavepackets evolving on different electronic surfaces. 2) At later times, the coherence disappears completely due to the increasing position separation in the low-frequency modes. 3) In agreement with another study, we observe that only normal modes that preserve the symmetry of the molecule induce decoherence. All together, we justify the enhanced charge migration by a combination of increased hole-mixing and the disappearance of decoherence contributions from specific vibrational modes—CO stretching in butynal and various H rockings in pentynal.