Toro Rosso struggled early on with straight-line speed, mainly as a result of its year-old power unit supplied by Maranello.
A new aero package was designed and introduced in Germany over the summer, but Tost admits the team's performance could have gone from bad to worse.
"We risked a lot," Tost told Autosport.
"We had to risk a lot and we came up with a completely new aero package. We had already a very good car on the aerodynamic side.
"With a new aero package we got more downforce and the car was better, but of course if you have more downforce sometimes there's a little more drag.
"The others made some progress with the power unit and everything together meant that we simply didn't have any more of a chance to be permanently close to them.
"We couldn't catch up any more, it was too much. There was no way to develop the chassis to a level that we successfully could compete against the other teams where the power unit as well was developed."
By no means is Tost critical of Ferrari's supply of engines in 2016, even if the team has chosen to race with Renault power in 2017.
"It had nothing to do directly with Ferrari's power unit as it didn't drop off," said the Austrian.
"It was always the question mark for me because you must see that the output from the power unit was always the same, all over the season. Ferrari did a fantastic job, was always the same."