You've conflated two different types of ship.
LPH - Landing Platform, Helicopter. HMS Ocean is the only purpose-built example of this class of ship these days, though some carriers have either been built or modified for the role (e.g. Cavour, HMS Illustrious - & the CVFs will be), & any carrier can do it, though less efficiently. An amphibious helicopter carrier. Meant for launching heli-borne assaults. No dock. Any landing craft have to be carried on davits.
LHD - the D stands for Dock, & that's the crucial difference between them. A through-deck amphibious ship with a helicopter hangar & a dock for landing craft, either conventional, or air-cushion. The USN is the largest operator of such ships. Juan Carlos & Mistral are of this type.
The Mistral class is about the same size as the Invincible class carriers. Cavour (one of the ski-jump carriers) is larger, bigger than Juan Carlos.
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