I've seen this word used as the opposite of magnify, but it's been in specialist fields. That word is minify. However, dictionaries list this word. It seems to have come about analogously using magnify as a model:
To reduce in actual size or amount; (now) especially to reduce the size of (an image).
From classical Latin
min- + -ify, after magnify.
Oxford Living Dictionaries
To make smaller or less significant; reduce.
American Heritage Dictionary
to minimize or lessen the size or importance of
from Latin minus less; for form, compare magnify
1.to make less.
Latin min(us ) less + -ify, modeled on magnify
Random House Unabridged Dictionary (dictionary.com)
2.To reduce in apparent size, as for example objects viewed through a lens or mirror shaped so as to increase the field of view, such as a
convex or aspheric mirror or a Fresnel lens.
redirected from minify
A reduction in the
apparent size of an object. Example: viewing a distant object through
the objective of a galilean telescope. Syn. negative
Dictionary of Optometry and Visual Science
Magnification, by a factor of less than one,
producing a smaller image.
Wikipedia magnification disambiguation
Contact lenses neither magnify nor minify the image because they are
placed on your cornea.
Physics of the Human Body, 2007
The most common types are reverse telescopes ... which minify objects
being viewed but present more information by means of the enlarged
Dictionary of Optometry and Visual Science, 2014
Figure 3.13b is a photograph taken through a 35-mm lens which minified the optic array by a factor of .70 ...
The Perception of Pictures: Alberti's window, the projective model of pictorial information, 1980.
Some lenses magnify, others make things smaller (minify). How much they magnify or minify can be calculated, but we have to know the actual numbers in your prescription(s)...
From a reddit forum post on optometry board
The lenses in your glasses can magnify or minify the look of your eyes. WHY?!
Facebook post from Adra Eye Care
It also comes up in thesauruses. However as you can see, it seems to be a term that's arisen after magnification to mean its opposite, and is probably not often used outside of specialist domains. In light of that, and since you don't seem to writing a technical or academic piece, you may want to use a more common English word.