Manchester United drew 2-2 against Burnley today to further dent their title hopes.
Both United and City spent big money in the transfer window last summer.
City can move 15 points clear of second-place United in the league table if they beat Newcastle tomorrow.
It was put to Mourinho after the match that he spent almost £300m on new players.
But he replied: “Is not enough, is not enough.
“And the price for the big clubs, the price for the big clubs is different from the other clubs, so the big, historical clubs are normally punished in the market for that history.”
He added: “When you say a club like Manchester United, do you think Milan is not as big as us?”
“You think they are not as big as we are? Do you think Real Madrid are not as big as we are? You think Inter Milan is not as big as we are?
“There are many big clubs and you say big clubs, I know what is a big club.
“One thing is a big club and another thing is a big football team that you know is not one of the best teams in the world, so when you speak about responsibilities to win the Premier League, Tottenham does not have that responsibility.
“Tottenham is not a club with the same history as us. Arsenal doesn’t have the responsibility to win it, Chelsea doesn’t have the responsibility to win it.
“Manchester City buy the full-backs for the price of the strikers, so when you speak about big football clubs, you are speaking about the history of the club.”
The message to the United board ahead of the January transfer window could not be more clear.
Mourinho is not expected to spend big next month but some changes to his squad may be made.
United went 2-0 down to Burnley at Old Trafford through goals from Ashley Barnes and Steven Defour.
But Jesse Lingard’s second-half double earned the Red Devils a point.
The draw comes after a point was picked up at Leicester on Saturday, while United were also dumped out of the EFL Cup to Bristol City last midweek.
“We should have won, yes,” Mourinho said of the match.
“I take satisfaction by the way the team played, by the way the team reacted, to try to recover from 2-0 down.
“The boys they fought against the 2-0 at half-time, so nothing at all negative to say about my players.
“I praise their spirit, their reaction, the way they accept all the risks we took in the second half.”
Mourinho also refused to blame fixture congestion for the result.
“It was the same for us and the same for them, they are also one of the teams that is punished by the fixtures,” he said.
As for Sean Dyche, the Burnley manager was left to rue a missed opportunity to take three points from Old Trafford.
“A lot of pride in a team that is stretched at the moment,” he said.
“The game plan is you’re going to have to defend and do all of the ugly details of the game well.
“We scored early, we didn’t relax on that, we scored again. Second-half they dominated a lot more. To come out with a point is pleasing.”