Once more, with feeling...
Jango's helmet is smaller than Boba's, as Jango was shorter than Boba by several inches. The entire rest of Jango's armour is different from Boba's, too -- plates, sash, vest, flight suit, gauntlets, etc. Jango's helmet has a longish oval dent up on the crown over the left ear running front-to-back. Boba has a round crater down on his forehead.
But if that ain't enough, in the Clone Wars animated series -- which, much as we might not like it, is as canon as the films in Uncle George's eyes and beyond contestation -- Jango's helmet has been destroyed. It is an ex-helmet. Boba does not wear his father's helmet ever. If he scavenged it after the Battle of Geonosis, it was was to use in his plan to kill Mace Windu with a thermal detonator hidden inside it. He will start wearing his own partial Mandalorian armour in this season, but the helmet he will eventually get is not Jango's.
The dent in Jango's helmet was put there as an homage to the crater in Boba's. Nothing more. *sigh*
--Jonah