mvictor
I went for B because a small fraction still might be few thousand of employees...
say company has 100k employees
out of them, only 10% are going there..
that's 10k people!!!! still a big number

Hi mvictor,
Thanks for weighing in. As I'm sure you've seen,
the GMAT is infatuated with the logical implications of the Number vs. Percent concept, namely this:
The number isn't important, it's all about the percentage (i.e. "A lot of people do x", but a lot relative to what?). Let's walk though the facts in the prompt, and then take a closer look at option B to see how it undercuts one of the core thrusts of the argument:
From the prompt:
The petition also claims that Craverton has
the greatest number of employees of any company in the city, and that closing the exercise facility would leave that sizable workforce without an alternative for something as important as personal fitness.
Each of the following, if true, weakens the claims raised in the petition
EXCEPT:
Ⓑ Employees interested in the company fitness facility make up a disproportionately small fraction of Craverton’s workforce.
The size of the actual number is irrelevant in the context of the proportion of workers who use the facility. Yes, there could still be a large number who use it, but based on the facts of the prompt, that's a significant proportion of Craverton employees, let alone relative to the proportion of citizens in general.
Even with your example that there are 10K employees interested in the facility, the actual number is meaningless to the logical force of the argument, which rests on the small size of that fraction relative actual number of Craverton employees (and the number of employees in the city), which in percentage terms is not as significant as the argument attempts to purport; thus, this option provides a weakening force to the argument, and since this is a Weaken EXCEPT question, we dump the option.
In sum, you related this question to an important concept on the exam. Thanks for your inquiry.