You may find that text changes size when it includes a variable reference, depending on the length of the variable's current value. This is by design to ensure that the text is always visible to learners, no matter how lengthy it is.
For example, let's say you're using a variable to display text, then you update the variable with new text at a specific point on the timeline or during an interaction. Depending on the length of the text, it may shrink to fit within its container.
To avoid shrinking variable text, make the text box big enough to accommodate the longest variable value.