Learn English – Word meaning the opposite of “negligible”

I believe that infinite is the opposite of infinitesimal.

Infinite : extending beyond, lying beyond, or being greater than any preassigned finite value however large

Infinitesimal : taking on values arbitrarily close to but greater than zero

So while infinitesimal talks about values arbitrarily smaller than any finite value but greater than zero, infinite talks about values that are arbitrarily larger than any finite value however large.

There's the term world's smallest violin, which is used to express mock sympathy.

In conversation, it's often accompanied by a hand gesture, as if someone is playing a miniature violin. For example, to that "A" student complaining about his B on the math test, I could say something like:

Aw, that's a shame. Here's the world's smallest violin, playing I Feel Sorry for You.

The website TV Tropes says this about the gesture:

A predominantly Western gesture, the World's Smallest Violin is given to a person who is exaggerating the sorrow of his/her predicament in order to gain sympathy; the person who does the giving presumably has none.

Wikipedia mentions it under a list of hand gestures; the page there reads:

World's Smallest Violin (also called "How Sad" or "World's Smallest Violin Playing Hearts and Flowers") is made by rubbing the thumb and forefinger together, to imitate bowing a violin. This gesture is used to express sarcasm and lack of sympathy, in response to someone exaggerating a sad story or unfair treatment.

Another (more brief) way to express such mock sympathy would be:

Oh, “Woe is me!”

I put the quotation marks in that sarcastic remark, because if I said “Woe is me!” in that vein to the B student, I would be putting those words into the student's mouth.