Cause you weren't paying close enough attention. For example The Titan-A has every ship that bore its name up on the wall in the ready room, a Shangri La, a Luna, and a Neo-Connie, we can infer that Neo-Connie is bigger than Shangri La because all of 25th century starfleet's throwback classes like the Sager class vs the Constellation class are also clearly larger then their inspirations. I could go on but you'd clearly be better served by going back and reviewing on screen details yourself.Funny I don't remember seeing any of that on the screen. Star Trek Cannon is only what's on screen. So that's what I use when making my arguments.
Meanwhile you have nothing to support that Excelsior is older than TSFS other than they neglected to bluntly say "its amazing captain DID YOU KNOW THIS CLASS DIDN'T EXIST BEFORE X DATE!?" there are no hints to its design language or progenitors before that movie it can only be infered to be a transformative generational step until new on screen information to the contrary is presented.