Jtdylktuy

I think you may be looking for the verb "to shrink".

The balloon shrinks/shrank/will shrink/has shrunk/is shrinking etc.

I wouldn't use that about something that's actually having bits taken out of it, though, because it tends to suggest getting smaller while retaining largely the same shape - or at least changing shape in some smooth, continuous way.

Reduce can be used without 'in size' for some things, and would often be used as such as about prices. Also about swellings, thrombosed haematomas, and various other things.

In addition to shrink and contract mentioned above, other words that can mean "shrink" plus some additional information or context are:

• 
  shrivel - shrinking by losing something (like water)


• 
  wilt - similar to shrivel


• 
  collapse - shrinking by losing its structure


• 
  deflate - shrinking by losing its content


• 
  implode - similar to deflate but more dramatic


• 
  retreat - like shrink, but focuses on the space where it is not anymore


• 
  recede - similar to retreat


• 
  wane - similar to shrink

There's also diminish:

to make/become smaller; to lessen the authority or dignity of; to disappear gradually.

The word "shrink" would fit most of the situations you ask about.

A balloon shrinks as it deflates

An apple shrinks as it is eaten (slightly odd but okay)

The price shrinks in a sale

My jumper shrank in the wash.

and so on.

If I were to choose a verb to satisfy your needs I would certainly say to shrink. Yet, the verb to contract can be used as a synonym in some situations.
According to The Free Dictionary, definition#2:

2. To become reduced in size by or as if by being drawn together: The pupils of the patient's eyes contracted.

An old word is

wane

which these days is only applied to the cycles of the moon (specifically waxing and waning), or metaphorically to something which changes size over time in a similar way to the moon.

Another word I see in the academic and statistics literature is the word attenuate. For example

The estimated association between the drug treatment and the disease
condition was attenuated after controlling for age.

That is, the magnitude of the association became smaller.

Depending on the target of the sentence, "drop" can also be used; for example

"The price is dropping"

"The size of the apples have dropped"

One however would not say

"The apples have dropped"

Since the size of the apples is not the target, but the apple. Note how the price example does not need any mention of "size"; (which is why I mention it) though I'm really not sure why and will update this answer if someone can explain in the comments.

To discount is to reduce the price.

To diminish is to shrink in size or importance.

An apple gets smaller (when somebody eats it)

To be eaten and to become smaller, for a discrete object like an apple, or not the same thing at all. You can say (these are not common, but they are grammatical)

• the amount of edible apple shrank


• 
  little apple was remaining
  

In these cases we focus on "apple" as a foodstuff, by adding "edible" in the first case and "little" in the second.

If you say "the apple shrank," I definitely picture an uneaten apple, and I may infer it shrank because it shriveled or dried up since this is the most natural way an apple may shrink, but I do not believe it was partly eaten.