These are Painted Lady (Vanessa cardui) caterpillars. They’re the same age and same species but some have noticably smaller heads than others. Why is this? Just curious.
Contrary to popular belief, the soft parts of many insects can in fact grow without molting.
In Lepidoptera freshly molted individuals have relatively large heads and thin bodies; vice versa for individuals that molted a long time ago. Their bodies gradually fatten up in between molts (this is why most caterpillars tend to be wrinkly, the wrinkles allow them to expand accordion-style as the animal gains mass).