Q:

Why so much Red Matter? 

In Star Trek (2009), the villain Nero uses Red Matter in order to 

 Create a singularity at the center of Vulcan, destroying the planet. 

This Red Matter was initially created by Spock 

 in order to create a singularity to destroy the Hobus Supernova. 

In both cases it clearly only takes one drop of Red Matter in order to achieve this.  Despite this, a significant amount of Red Matter is seen in Spock's ship- much more than was needed simply to 

 Destroy the Supernova 

(Not sure if that needs to be in a spoiler box, but it's a pretty major plot point.) 
So, in short, why did Spock manufacture this:

When he only needed one of these? 

(I'm looking for an in-universe answer; out of universe I understand that there's no plot without the extra Red Matter)

A:

In-universe
The Vulcans don't seem overly keen to have a potentially world-ending substance sitting around in their science facility.  Spock and LaForge note (in the Countdown prequel comic) that the substance is inherently unstable and that the custom-build container on the Jellyfish is simply the safest place to keep it. 

Having loaded all the red matter onto the Jellyfish the endgame seems to be to jettison whatever's left into the singularity formed, neatly disposing of it in an environmentally friendly manner. 

Out of universe
The existence of a giant red ball is an in-joke between the Director (Jar Jar Abrams) and the film's Production Designer (Scott Chamblis). 

I was really happy with the way the Bridge turned out.  I guess I have
three favorite sets: the Bridge, the whole Narada interior, which is
minimalist, with excessive detail, and brutalist, and the Jellyfish,
Nimoy’s little ship [the ‘Jellyfish’].  That has a wonderful inside
thing from J.J. and my years of working together.  In the containment
field in the ship is a big red ball.  The ‘big red ball’ has a lot of
resonance for J.J. and I: we have one in virtually everything we do. 
It started with the Alias pilot.  I always look at a script and wonder
what the big red ball is going to be this time. 
Interview with Scott Chambliss, production designer on J.J.
Abrams new Star Trek movie - Star Trek Magazine #17

and

QUESTION: Everything you do has a big red ball in it. 
JJ ABRAMS: Not everything.  There are a couple of shout outs to some of the stuff that we've worked on before and one of them is the
big sort of Rambaldi ball from Alias, which was also a cameo. 
JJ Abrams Star Trek Interview - Femail

