Length of Matlab Raised Cosine Filter Output Complex Vector
The filter computes OutputSamplesPerSymbol output samples for each input value. You would get the complete convolution if OutputSamplesPerSymbol were equal to $1$ and if you added zeros to your input signal until the filter memory contains no non-zero input values anymore.

This is the standard way a filter routine works; if there is no interpolation or decimation, it computes one output sample per input sample because usually the complete input signal is not stored in a buffer, but it arrives either sample-per-sample or it is split into several blocks.