No, not necessarily. The fundamental difference between an indicative statement and a modal statement is not in strength. The difference is in the factuality of an indicative statement and in the intent, conditionality, or probability of the modal statement.
Among the modals themselves, however, there are definitely differences in strength--for instance between "will" and "would" and "shall" and "might." We can see and feel this difference in strength between "I will do it no matter what" and "I would do it if you tell me to do it," and between "I shall return" and "I might return." The strength of intent, conditionality, or probability is what constitutes the modality of the statement.