Most restaurants have a variety of seating available, in such a way as to accommodate both large and small groups. Smaller restaurants clearly have fewer choices. Consider that in your small restaurant the best/possibly or sometimes only way to accommodate say a group of ten, is to combine tables, 3,5, and 6. Trying to accommodate it anywhere else, obstructs the aisle, blocks the fire exit, or makes the other tables awkward to serve.
You call on Monday and the three of you fancy table 1, as it’s it the window, or whatever. A server possibly took your reservation, on a fully unreserved floor plan, you can have any table you fancy, it would seem.
But come Wednesday a group of ten wants to book a table. What to do? The answer is relocate the table of three so as to accommodate both groups. Unless they moved you to beside the bathroom, and especially if it’s a busy Fri or Sat, the patron should try to be understanding. Turning away a table of ten to accommodate three would be silly. One assumes if you like the restaurant, you’d like it to succeed.
All that said, most restaurants insist on a phone number when making a reservation. That means it should play out such that the manager calls you, well beforehand and explains that the person who took your reservation was uninformed/new/in error, etc, they should politely explain and assure you they’d like to make it right, with another well situated table, or free appetizers or desserts or some such.
The person who took your reservation, should have told you it was open BUT you may be moved if a large group need be accommodated, if that’s the case. At the very least you should be informed before you turn up at the restaurant.
Yes, you should complain to the manager, as they definitely should have handled it better. But consider if you’re a small group requesting a larger table that you’ll be more likely to be accommodated on a Tuesday than a Friday or Saturday.
If it helps consider that the train wouldn’t let you take up four seats for two people, just so you’re more comfortable or have room to spread out. If the trains empty, sure. If it’s busy though, not so much. Restaurants are in a similar position!