Analog sources that are processed with Pro Logic follow the same bass management logic as digital sources. That means that for large mains and the sub set to l/r+sub, the mains get a full range signal and the sub gets low frequency data from any small speakers plus the low frequency data from the fronts - exactly as you describe it. Even though the fronts are set to large, the sub gets low frequency data from those channels, as is intended for the subwoofer setting of L/R+Sub. Setting the subwoofer to LFE in that case would prevent this (the sub would only get low frequency data from small speakers in this case).