Fair enough that anime doesn't take real life physics into account, but surely something as large/long as a few hundred metres can't make a complete turn in less than 1 second. The Orchis is approx 70m without the rifle barrel and it probably is much quicker than a battleship; with its thrusters, it definitely has a greater acceleration ability than a small MS. Maneuverability wise, it has to move its bulk in big arcs as shown when it's dodging missiles... It cannot simply avoid shots by quick moves shown by the Zakus in Igloo2 where it skips aside to avoid the shells of a Type 61 tank.
Going back to the issue of giant machines, the Neue Ziel IMO is a better equipped weapon than the Orchis. All of Neue Ziel's weapons can all fire in various directions simultaneously. The Orchis on the other hand can fire all the missiles, rifle, plus two weapons of choice (rifle/bazooka) held by the Stamen's arms; but it's all aiming forward. Granted, the missile pods open up and scatter, but it has to be done from a distance otherwise the Orchis itself will be caught in the explosions.
Maybe that's why EF decided to forsake the Orchis... and also the consideration of high costs, plus the beautiful excuse of sweeping the GP project under the carpet. And the Titans decided to waste resources on Psycho gundams which had massive destructive power but horrible agility. Then again, 0083 was produced after Zeta so yeah.
As for Zeon, they just continued to make gigantic MAs for the fun of it... I haven't watched ZZ to know how effective they were, but Neo Zeon's forces had quite a number of large sized MS/MAs... In CCA, everything just went plus-sized... normal MSes reached the near-20m mark, and those fitted with psychoframes exceeded it. Alpha-Azeiru was huge, yet it's armaments are pretty much standard stuff for Zeon MAs... maybe the size is just for intimidation purposes. During 0093 the mecha's performance all increased exponentially compared to early UC, so AA might just manoeuvre faster than Orchis, but smaller MSes also got quicker by then.
In 0096, everything was still pretty large with Kshatriya and the new Shamblo... will we get another bigass MA for Zeon or even EF for the finale?

We all know how the smaller, more agile Xbone suits thrashed the bulky EF suits in F91, but of course pilots do play a role here. EF probably got complacent with no real rival in the 20+ years after Zeon surrendered.
I generally agree and prefer the Neul-Ziel in terms of balance between ship killer and as anti-MS weaponry. Dendro is like a fast interceptor one-manned cruiser class ship whereas Neul-Ziel is jack of all trades.
LOL nyoron gets pwned.