Agree that the party size is too small. Disagree that it's DOS's fault.
Ironically, I like to play tons of solo, but IMO having a higher party limit is usually better than a smaller one, since it offers more player choice.
The down-sizing of party members in "mainstream" CRPGs started way before DOS. I'm not sure why (maybe technical issues? Maybe to market "replayability"?), but we saw years of party size getting smaller and smaller. NWN 1 limited you to 1 henchman (2 in HoTU), Kotor - 3, Dragon Age - 4, NWN2 - 4 (IIRC), Shadowrun - 4, etc. We saw a resurgence of 6 with POE 1 and Kingmaker (7 if you count Wasteland 2), but even then Obsidian downsized to 5 in POE 2, and 4 in Tyranny.