When you construct a submodel envelope around part of a model diagram, then the symbols inside it are the same size as the ones outside. Similarly, if you construct an empty submodel, then the symbols you place inside the submodel are, by default, the same size as symbols you place outside. In principle, one could construct a complex model, containing many (perhaps nested) submodels, with all the symbols being the same size at any level of nesting. In practice, however, this may be undesirable or difficult to achieve.
You can control the scale of the symbols inside the submodel relative to those in the desktop by using the "Relative scale" slider control in the "basic" tab of the submodel properties dialogue box. This dialogue box is displayed by double-clicking anywhere within the submodel, or by selecting the "Properties…" command from the context menu, which can be displayed either by right-clicking in the submodel or by opening the submodel in a new window, and using the "Edit" menu in this window.
The default symbol size is the same as the desktop, i.e. a relative scale of 1, and using the slider control it is possible to reduce this to make the symbols smaller. If you wish to make the symbols in a submodel larger than in the desktop, you must use the same slider control on the desktop window, either by double-clicking on the desktop or selecting the "Properties…" command from the context menu. This control allows you to reduce the absolute size of the desktop symbols, meaning that the symbols in submodels are now relatively larger.
Note that if you open a separate window to display the submodel's contents (e.g., by doubleclicking on its border) and then do zoom operations in that window, these may also affect the relative scale of the submodel contents. For instance, if you "zoom out" and this results in more whitespace being added around the components, then the scale is reduced because the extra space has to fit inside the same submodel envelope in the parent. This may also happen if you expand the separate window to display more space. Conversely if you "zoom to fit" in the separate window, this may trim whitespace from the submodel contents, causing their relative scale to increase so they fill the submodel envelope.