How Exactly Does MATLAB Zero Pad Signal?
Lets say you have a vector $ x = {\left[ 1, 2, 3, 4 \right]}^{T} $.
You want to have a look on its DFT transform then you apply DFT on it and have the 4 points DFT transform of the data.
In MATLAB it would be something like:
vXDft = fft(vX);

Yet, what happens if you want the 64 point DFT of the same data?
In MATLAB it would be something like:
vXDft = fft(vX, 64);

So what happens?
MATLAB just adds zero elements at the end of the vector until its size is 64:
vXDft = fft([vX; zeros([60, 1])]);

This will match what happens above (I assumed Column Vector).
In practice you get a denser grid in the Frequency Domain (64 Points).
Where the points are interpolated by Dirichlet Kernel (The Discrete equivalent of the Sinc function).