Washington Gas Light Co. has proposed changes in its Maryland rates that would result in lower bills for all residential customers and small business customers - and higher bills for users of large amounts of natural gas.

The savings would be $40 a year for a small residential customer and $204 for a commercial customer using a small amount of gas, the company estimated.

Under the rate structure proposed Friday to the Maryland Public Service Commission, WGL's total revenues would remain the same, the company said.

Most of the small customer's savings would come from a lower system charge, the fee that WGL began charging its Maryland customers in November 1976 to cover some of its fixed costs.

The proposed system fee for a user of less than 1,000 therms a year would be $5 a month, compared with the present monthly rate of $8.20 for a residential user and $23 for a commercial user.A therm is a measure of the gas's heat content.

WGL has made similar proposals to change its system charge in the District and Virginia. It also is in the midst of requests for additional annual revenues of $11 million in the District and $8.3 million in Virginia.

The proposal made in Maryland is part of that commission's investigation of WGL's rate structure, which will involve public hearings next month before the commission reaches a decision.

To offset the lowered revenues from the decrease in the system charge for small users, WGL proposes to raise the per therm charge for commercial users. Under the present rates, commercial users pay about 23 cents per therm while residential users pay just over 25 cents. The proposed changes would result in one per therm rate - slightly less than 25 cents - for all users.

Thus the proposed rate structure would result in lower bills for small users mostly though a decrease in the system charge, and higher bills for large users through higher per therm rates for them.

Small residential heating customers would see a 12 to 31 percent decrease in their bills, and small commercial customers would see reductions of 39 to 72 percent. WGL assistant vice president Edmund Smallwood estimated.

WGL originally proposed a monthly system charge of $10.50 for residential users but lowered that to $8 under pressure from People's Counsel John Keane, who said the charge should be lowered further to $4 a month. The Maryland Action Coalition, a consumers' group, has also called for a reduction in the system charge.

Under WGL's proposal the system charge for large commercial users would remain the same at $23 a month compared to the $5 rate proposed for small users. The proposed system charge for users of 1,000 to 3,000 therms would be $8 a month, compared with the $23 now charged commercial users of that amount and the $8.20 now charged residential users.

Consumer groups have protested WGL's use of the system charge since it was inaugurated in November 1976. They charged that WGL was "ripping-off" customers by charging them two rates for gas usage rather than the one rate used before.

WGL has asserted that the system charge is needed for the company to pay its fixed costs which remain constant during the year, while gas usage varies with the weather.