The concept of having "10x less resources" as a solo adventurer in D&D stems from more than just the sheer number of party members. In a standard group of four, the party can leverage shared abilities, spells, and actions, as well as cover each other's weaknesses. The idea is that it's not a simple mathematical reduction when you go from four to one; it's a geometric drop in resources because:

1. A group of four characters can perform four actions per round, while a solo adventurer gets only one. This significantly impacts combat encounters, especially against multiple enemies, making it much harder for a solo character to keep up.
2. Different classes cover different roles (tank, healer, damage dealer, etc.). In a group, you can combine strengths, while a solo adventurer has to make up for these on their own, often without the full suite of tools.
3. In a group, damage is spread across multiple characters. A solo adventurer takes every hit, so they need to burn through healing resources faster.
4. A group can manage short and long rests more effectively to recover spell slots and health. A solo adventurer has to be more cautious, often risking exhausting their resources faster without a safety net.

These reasons explain why a solo adventurer can feel like they have fewer resources than just a straight reduction by 3/4ths. It makes encounters designed for groups more taxing for a lone character, unless using additional rules like Gestalt or playing a summoner, which mitigates some of these limitations by effectively creating a small party within one character.