Which Vienna Convention agreement was India expected to break?

Question

[Credits for the articles (a law.se post): What is the legal way to expel diplomats from a country under the Vienna convention? ]

A diplomatic spat is ongoing between India and Canada.

India sought parity in diplomatic presence with Canada and asked 41 of Canada's 62 diplomats to leave - and threatened to remove diplomatic immunity if they didn't leave by the deadline (so essentially the diplomats were given about a month's time to leave). Meanwhile, the western countries (US, UK, Canada) are urging India to "uphold its obligations under the 1961 Vienna Convention on Diplomatic Relations".

afaik, I didn't see any agreement violated. I found that the Vienna convention on diplomatic relations has the following articles that allow for expulsion of diplomats and removing privileges. Then, what exactly do the western countries mean here?

9.1 The receiving State may at any time and without having to explain its decision, notify the sending State that the head of the mission or any member of the diplomatic staff of the mission is persona non grata or that any other member of the staff of the mission is not acceptable. In any such case, the sending State shall, as appropriate, either recall the person concerned or terminate his functions with the mission. A person may be declared non grata or not acceptable before arriving in the territory of the receiving State.

11.1 [This one was even cited by the Government of India] In the absence of specific agreement as to the size of the mission, the receiving State may require that the size of a mission be kept within limits considered by it to be reasonable and normal, having regard to circumstances and conditions in the receiving State and to the needs of the particular mission.

39.2 When the functions of a person enjoying privileges and immunities have come to an end, such privileges and immunities shall normally cease at the moment when he leaves the country, or on expiry of a reasonable period in which to do so, but shall subsist until that time, even in case of armed conflict. However, with respect to acts performed by such a person in the exercise of his functions as a member of the mission, immunity shall continue to subsist.

Answer 1

FWTW, the UK was slightly more detailed:

Britain's Foreign Office also cited the Vienna Convention. It said "the unilateral removal of the privileges and immunities that provide for the safety and security of diplomats is not consistent with the principles or the effective functioning of the Vienna Convention."

I'm guessing the quibble is that unlike paras you've quoted, which deal with declaring a diplomat persona non grata, India didn't do that, but declared that the (expelled) diplomats are not diplomats anymore, after a certain date. I'm not sure if there's practical distinction between these cases. But anyhow, Canada itself claims that

Diplomatic immunities should be respected and cannot be unilaterally revoked by a host country.

Or perhaps they implicitly argue (with that "effective functioning") that Canada needs more personnel to process visas for Indians, given e.g. the number of Indian students travelling to Canada. These generally need more paperwork than tourist visas. AP says:

Of the more than 800,000 international students in Canada at the end of 2022, 40% were from India, according to Immigration, Refugees and Citizenship Canada data.

And the Canadian announcement said that

Unfortunately, this mass expulsion will impact our operations, and client service will be affected. We will now be forced to pause temporarily all in-person services at Consulates, until further notice.

Canada seems to have 7 consulates in India. Vice-versa, India seems to have only 3 consulates in Canda. Of course, India is entitled to parity (or even to break diplomatic relations altogether), but whether that's reasonable...