Boston-area tenants pay a bigger chunk of their income in rent than tenants in any other major city in the country, while Houston residents pay the smallest, a study of 25 cities said. A California research firm, Sound Advice, said that Boston’s median rent is $586 a month, about the same as San Francisco, but the study found that median incomes were lower in Boston. Thus, while San Franciscans pay 19.5% of their income on rent, Bostonians pay 21.3%. Next to Boston, the least economical city was Los Angeles, where median rents were $521, or 20.7% of income. Renters in New York pay a median of $444 a month. Although the New York rent figure is lower than San Francisco’s, it amounts to about the same percentage of income as renters in San Francisco pay, because New York incomes are lower, the study found.