Maximalism is, generally speaking, the idea that more is better. It often incorporates the idea that qualitative improvements are dependent on quantitative increases – that "progress" is not even possible without quantitative growth. Maximalism is prominent in many areas of modern technological civilization but is particularly so in computing, thanks to Moore's law.
Maximalist aesthetics strives for the maximization of resolution and detail. This leads to increased screen sizes and growing bandwidth requirements.
Maximalism leads to an increasing use of artificial energy and other limited resources, even when the energy efficiency is dramatically improved (see Jevons paradox). This is extremely unsustainable, so permacomputing prefers to take an anti-maximalist, "small is beautiful" type of stance that emphasizes qualitative improvements and technological diversity.
Extropianism, as advocated by the transhumanist philosopher Max More (sic), is an extreme philosophical stance based on maximalist ideals. It dreams of things such as human immortality, quantitative intelligence maximization and an endless expansion of the maximization-oriented "civilization" into the outer space.