What does the term 'spare habit' mean exactly?

Example in Usage:

July 20, 1890. Consulted by Mrs., 50, spare habit, dark hair and eyes, nervous temperament, neurotic family history;...

Solution 1:

Spare habit here means thin body.

A DICTIONARY OF ENGLISH PHILOSOPHICAL TERMS - Francis Garden - 1878

In physical and medical science, the word is used in its original largeness of meaning, i. e. not as confined to action, but as embracing states, modes of being, and we are accordingly familiar with the phrases "habit of body," "a full" or "a spare habit."

So habit can be used to refer to the body. For further clarification, we can consult:

HABIT IN THE ENGLISH NOVEL, 1850-1900: Lived Environments, Practices of the Self.

Outward form or general appearance is a 'habit'... I may be of a spare habit whereas my friend is inclined to corpulence.

The author is contrasting corpulence with spare habit here.

If we consult a dictionary for the word corpulence, we discover it means:

The state of being fat; obesity.

-- O.L.D

Solution 2:

Spare means thin, with no excess fat and habit means constitution.

I may be of a spare habit whereas my friend is inclined to corpulence

Habit in the English Novel, 1850-1900: Lived Environments, Practices of the Self