The decline isn't that big if you adjust for seasonality.
Literal
Decline [topic-は] seasonal-adjustment [if-ば] that-much big is-not.
季節調整 ('seasonal adjustment') is economics jargon for the statistical technique that removes seasonal patterns from data. すれば is the ~ば conditional of する. それほど + negative = 'not that much / not so much' — a fixed pattern for understatement.