Pep Guardiola's Premier League leaders spent in the region of £220 million in the most recent transfer window as they completed an overhaul of the playing squad.
United, behind City only on goal difference, invested close to £150m on Romelu Lukaku, Nemanja Matic and Victor Lindelof as they continued their recent trend of massive pre-season spending.
Spurs, by contrast, spent just over £90m and ended the window with a profit, having sold Kyle Walker to City for close to £50m among a number of departures.
The club are also directing huge investment in the construction of their new stadium on the site of White Hart Lane, which is due to be ready for the start of next season.
But Pochettino says there is a marked difference between the way the Manchester clubs are able to spend and how others must be more cautious.
"The difference these days between clubs in the Premier League is that there are clubs, like City and United, who sign who they want and when they want," he told beIN Sports.
"Then there are big teams, like us, who sign when we can and who we can. It's the big difference today."
Spurs, who are five points behind City and United in the table after seven matches, host Bournemouth on Saturday.