Most variegated plants tend to have less vigor than their non-variegated cousins, the possible exception being hostas :-). Usually this just means they grow somewhat slower and may be less inclined to bloom heavily but they shouldn't be shrinking in size!!

I am assuming these are something like Hydrangea macrophylla 'Mariesii Variegata' or similar as I am unaware of species other than bigleaf hydrangeas that come in a variegated form. In a zone 5 climate, these will need winter protection to avoid cold damage and significant dieback, as will just about any macs. That is most likely the cause of the "shrinkage" and not the variegation itself.

Unless you're growing Light O Day (and that's a maybe), you're whipping a dead horse in Z5. MV in particular is horrible. Bought mine in a 5-gallon years ago and it has never grown more than a foot tall. I gave it away.

Stunted growth could be a sign of circling root, aka girdling roots. The plants are also said to be 'root bound plants'. Normally, I would expect it to happen in year 2 or 3 but MV grows so slow that I would think it would last longer without stunted symptoms.

In zone 5, I would expect your MV to grow all stems from the crown in Spring unless you winter protect. If that is the case, I would not expect it to be as tall as it used to be last year until the end of the season, if ever.

Z5 Winter may also make it bloom unreliably if you do not winter protect. But then again, I grew it just for the foliage myself. The blooms were not that great to me. Not sure why variegated ones and yellow leaf-ed hydrangeas are like that in the bloomage department. They probably use a virus that does not kill the plant and causes variegation but weakens the plan barely enough to not excel in the bloomage or growth departments.

I am trying to grow the "Light of Day" and if it blooms that would be a plus.

If I can not get it to bloom the foliage would be fine.

Thanks for the winter protection tip.

What can be done to winter protect it?

Thanks

Do a search on the forum for 'winter protect'.

Basically, it involves surrounding the plant in chicken wire when it goes dormant in the Fall. Then you pile dried out leaves from trees inside or you use mulch. Top it with cardboard and rocks to prevent the leaves from flying away. Add more in mid winter if you notice settling. The ends of the stems develop invisible flower buds in July-August (Aug for most people in the northern states) and these are the guys that you want to protect so it will bloom when you remove the winter protection about 2 weeks after your last average date of last frost in Spring. The more leaves you pile on, lots and lots, the more protection they ends of the stems will get. Do the search for more info, The ideal "cage" shoul be wider and taller than the plant is wide and tall. Allow for the plant getting bigger if winter protection works and you get leaf out from existing stems in Spring as opposed to now getting dried out stems abd new growth from the base of the plant.