Personally I think only maybe a third or even a quarter of any unit's troops are in smaller-sized transports, these would be the first wave of the assault. The rest would be in battalion-sized dropships at least. And in the first place units would try their best not to conduct Overlord-style planetary assaults in the teeth of enemy defences.
I'd go so far as to say a good chunk is in cargo ships.
Take a typical mech regiment for example.
You don't need all 108-132 mechs in combat ships.
3 Leopards
3 Unions
1 Overlord
This gives you more than enough to drop from orbit.
The Leo's/Unions split up & hit strategic sites quickly while the Overlord/Unions drop into a future LZ to secure it.
Then once the ground is a secure-ish LZ the OL/Unions land & call down a wave of cargo ships with the rest of the regiment & support to off load.
Expand that up to an RCT & you get the same mech carriers but toss in an Excalibur and some Seeker/Triumph/Condors/Gazelles for full security at the LZ.
You also toss in a Fury or 2 out with the Leo's/Unions to add infantry to take those strategic locations.
When the Mules descend they unload the rest of the mechs but also regiments of Infantry/Vehicles from their cargo bays.
Each Mule can handle a Regiment of Grunts & Battalion of Tanks fairly easily and still have some cargo space.
3 Infantry Regiments as Cargo & 2 attached to the above combat ships up above gets you your RCT on the ground in good order.
7 Mech Carriers, another 10 Conventional Carriers, 5 Mules, and a Vengeance & 2 Avengers gets your DS count down from 30-50 DS for the RCT down to 25.
And honestly you could shave off 7 of those if you had too to squeeze just the first wave ships onto a Monolith & 3 Invaders & send the Invaders back for a 2nd round of ships.
4 JS (3 of the most common) to move an RCT invasion force in 1.5 waves isn't nearly the logistics tail that we've seen in some of the Novels/Sourcebooks where a dozen+ warp into a system.
I think the biggest issue is people trying to cram the entire combat force into combat carriers all in 1 wave w/ JS attached.
It would be far more likely to see 1/4-1/3 of the Mechs being cargo shipped & possibly over 1/2 of the Infantry & Tanks.
But that's just my thoughts on the matter.