United made it three games without a win in all competitions as they drew 2-2 with Burnley on Tuesday, with Jesse Lingard scoring twice in the second half to salvage a point after strikes from Ashley Barnes and Steven Defour before the break.
The result means City can move 15 points clear at the top of the table if they beat Newcastle United on Wednesday and leaves Mourinho's side facing what looks to be an impossible task in the Premier League title race.
Pep Guardiola's side have been in irresistible form this term following an investment of close to £225 million in the last transfer window, which included club-record spending on full-backs Kyle Walker and Benjamin Mendy.
City forked out roughly £191m in Guardiola's first year in charge, but United's spending under Mourinho has not been much below that of their neighbours, with close to £300m spent since he took over in May last year.
However, Mourinho feels United must still spend more to match City and are being "punished" in the market because of their illustrious history.
100 - Manchester City have become the first English team to score 100 top-flight league goals in a calendar year since Liverpool in 1982 (106). Superb. pic.twitter.com/FJGpa6u5nw — OptaJoe (@OptaJoe) December 23, 2017
"When you say a club like Manchester United, do you think [AC] Milan is not as big as us?" he told a news conference. "You think they are not as big as we are?
"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 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."
When reminded about his spending at United, he replied: "It's not enough, it's not enough. And 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."
Jose: "At 2-0, it makes things more difficult at half-time. I need my players to believe in themselves and in me. But I'm very happy with their character." #MUFC pic.twitter.com/ieG3CNUDfi — Manchester United (@ManUtd) December 26, 2017
Mourinho's comments came after a draw that means his side have taken only eight points from a possible 15 in their most recent Premier League fixtures.
But the former Chelsea boss insists he has been happy with the performance of his players on the whole in the stalemates with Leicester City and Burnley in recent days.
"The boys are doing what they can and they are doing fine," he said. "I say, last two matches, we had 15 chances to score and we concede three 'S' goals and an amazing free-kick.
"So the boys do what they can, they are trying hard with the problems and injuries we have. I am not happy with the result, and against Leicester, but I am happy with my players."