I feel that the pack's physical size and weight
does matter. That's what defines the battery's energy density.
The Nissan Leaf & Renault Zoe have had 3 "sizes" of battery pack (20/30-ish, 40-ish & 50/60-ish), but all were physically the same size and weight. The increased capacity was due to battery chemistry improvements over the past 15 years.
IMHO, manufacturers will use solid-state batteries in one of 2 ways.
- Increased range for a given battery pack size and weight
- Reduced pack size and weight for a given range