Let's look at some numbers.
The highest recorded weight for a blue whale was 173 tonnes They grow up to 29.9 m in length, but we'll go with some numbers from this archive. So what we have for our specimen is:
* Length: $\text{27.18 m}$
* Girth: $\text{13.90 m}$
* Weight: $\text{1,195,639.2 N}$ (assuming Earth gravity)
We can then use the circumference formula to find the width of our specimen:
$$C=2πr$$
$$\frac{C} {2π} = r$$ $$\frac{13.90} {2π} = r$$
* Radius: $\text{2.21 m}$
* Diameter $\text{4.42 m}$
For estimation purposes, we can now define a block of creature as being a rectangle 4.42 m wide and 27.18 m long. Now we can figure out how much area is covered by our specimen:
$$A=wl$$ $$A=(4.42)(27.18)$$
* Area: $\text{120.14 m}^2$
The pressure our resting specimen exerts on the ground is, thus:
$$P=\frac{W}{A}$$ $$P=\frac{1,195639.20}{120.14}$$
* Pressure: $\text{9,952.05 Pa}$
Glass, however, has a compressive strength of just $\text{1,000 Pascals}$.
As a result, our specimen would shatter its own scales when it was sleeping, let alone while it was moving around or lifting parts of its body off the ground.
<PERSON> raises the question of toughened glass, so I'll add a bit more detail.
On first viewing, the numbers for toughened glass look plausible.
* Compression Strength: $\text{69 MPa}$ (minimum)
But we'll need to dig a little deeper to see what it takes to produce toughened glass and whether our specimen can manage it.
There are two ways to produce toughened glass: physically and chemically.
Physical Solution (biologically interesting)
Physically toughened glass is placed on a roller table and sent through a furnace where it is heated to $\text{620 °C}$ then cooled via air flow. 441
This method has the advantage that our specimen can be constantly extruding new glass scales as the old ones chip and break. Unfortunately, we're going to need to employ some magic to have an internal furnace running continuously at that high of a temperature.
Chemical Solution (biologically challenging)
Chemically toughening glass involves immersing glass in a bath containing a potassium salt at $\text{300 °C}$ to let sodium ions in the glass be replaced with potassium ions from the bath. We still have that temperature problem, though, and the animal with the highest internal body temperature seems to be in the $\text{40-46 °C}$ range, depending on which source you want to use.
One fun side note about the chemical solution: You end up with a worm with a juicy, gooey interior. Strip off the glass armor and you have a gummy worm!
One last note: It might be possible to produce some form of glass compound using lead or another material to strengthen the glass (I'm having trouble finding numbers), but glass production is still prohibitively heat intensive and so requires the use of handwavium regardless. 561