Scaling up poorly known dinosaurs: a bad idea?

this was bugging the hell out of me. Why would world renowned, respected paleontologists just take an' educated' GUESS on a poorly known animals size, and just base it off of 'close' relatives? how can you sit there and say ' oh it was this big and this heavy when you have so little? My main example? Spinosaurus.

This is what they have found, a partial skull, a few tall spines, and teeth in sockets. really? how do you get this:

to this??

now, i've heard rumors they found parts of a leg bone not that long ago, and studies are being conducted on it. Can't find anything on this.

Now, i see why someone would use a 'scaled up' baryonyx or even perhaps suchomimus. Sure, you can argue all day it's a decent idea. It has a resemblance to these animals, yes. But say for example the komodo dragon was only known from skeletal remains let's say, a partial lower jaw, 9 inches long.

So using say a mangrove monitor, (something less long and skinnier, yet closely related) has a complete skull of around 10 inches.  scaling the whole body, and reconstructing the 'correct' skeletal structure, put The Komodo  dragon at 15 + feet and 600 or more pounds for an average! come on now...they get on average 10 feet and 300 lbs. so this is my point..The  truth is, we have next to nothing of this monstrous animal. A few photographs of some very partial remains. new finds are Even less then that picture above! I just don't see enough to support the suggestion of a 60 foot, 23 ton monster with a sail. you can't claim to know an animals natural ecosystem and what it did, or even what it looked like with such scant remains (as planet dinosaur did). We can do this with abelisaur, tyrannosaurs, or even megalosaurs because there has simply been more discovered. Size ranges established, niches classified. The complete opposite with spinosaurs.

This...is just my thought on the matter. I'm not hating on old spiny, and if you take offense, well...i honestly don't care. Feel free to leave your comments below, i would love to see what you guys think.