The Admiral class, Hood and Tiger's denser design

What caused the large jump from Tiger to the Admiral-class battlecruisers?

The original Admiral design pursued a much enlarged Invincible concept; post-Jutland changes moved it toward Tiger's approach without making it equally dense in armor and armament.

The explanation starts with Fisher's ambition to produce an Invincible concept that worked on a larger scale. The original Admirals are compared with the route later taken by the Lexington design. After Jutland, the design was reworked toward something more like Tiger. Tiger is described as a denser concentration of armor and armament relative to size, so simply enlarging her to Admiral dimensions would produce a heavier ship. This comparison underpins the argument against treating Hood as a fast battleship while continuing to call Tiger a battlecruiser. Renown and Repulse provide a further comparison: their standard displacements are stated to be several thousand tons below Tiger's.