At first I was inclined to agree with you, but after a little bit of analysis I'm not so sure anymore. Given the vastly inflated scale (both in weight and armament) of 40k vehicles to begin with, I don't think that "Titan Scale" ships are all that far out. In real life, comparing a main battle tank to a battleship in terms of size and firepower is
an awful lot like comparing your average 40k tank with a Titan. We build titans today in a manner of speaking.
I often find it useful to compare 40k (especially Imperial technology) to World War II era equipment, as it seems to follow that theme. Some of the largest, most heavily armed and armored tanks of the era are dwarfed by the battleships of the time:
King Tiger PzKfw IV Ausf. B
Main Weapon: 1x 88mm
Main Weapons: 8x 380mm
Armor: 110mm - 360mm
Main Weapons: 9x 460mm
Armor: 200mm - 650mm
In a sense, battleships were
real-life titans. We just needed to use water to get them to move around. I was imagining Imperial battleships that would make Baneblades look silly, but it seems that even real-life warships (more than within the reach of PDF forces) make baneblades look silly, and give titans a run for their money.
So lets scale up real life warships to a more impressive 40k scale.
WWII Era Medium Tank (Sherman, Panzer 4, T34) ~25-30 Tons
WWII Era Heavy Tank (Tiger, Pershing, IS-2) ~ 40 - 60 Tons
Modern main battle tanks (Abrams, Leopard 2, Merkava) ~ 60 Tons
Larger tanks have been built, e.g the Panzer VIII Maus (Largest tank ever, 2 Prototypes, never saw combat) ~190 Tons. This is a very strange outlier however, as no tank over ~70 tons has ever seen meaningful combat.
40k MBT (Leman Russ) ~ 66 Tons
40k Heavy Tank (Macharius) ~ 190 Tons
40k Super Heavy Tank (Baneblade) ~ 350 Tons
So your average 40k tank weights as much as the heaviest true land combat vehicles ever made. A somewhat less common heavy tank weighs 3x as much, and the relatively rare superheavy variety weights almost 6x as much.
WWII Era Battleship (Bismark, Yamato) ~50-70k tons, ~800ft
Modern Supercarrier (Nimitz) ~90k tons, ~1100ft
Largest ship ever built (Sumitomo Heavy Industries' Seawise Giant Supertanker) ~600,000 Tons, ~1500ft
If we use tanks to scale these real ships to 40k scale, you'd have 800ft Cruiser Class ships at 70k tons, heavier 1200ft battleships and carriers at 200k tons, and "super heavy" battleships, carriers, and various voidshield/defense laser-toting 2000ft behemoths at 600k tons.
I do see what you're saying though about the AdMech restricting access to these larger vessels. But I think its perfectly reasonable for PDF forces to have some of the "smaller" ships, while important campaigns and warzones get the massive leviathans.