The 24-year-old, who won the best and fairest award for Geelong's VFL side in the club's historic 2007 season, knew his opportunities with the premiers would be limited this year and beyond.
Competing for a spot in the first 22 with nine All-Australians, a Brownlow Medallist, the NAB Rising Star winner and the Norm Smith Medallist, Callan knew he would struggle to get regular senior appearances.
"You have to go where you'll get an opportunity and that wasn't going to be at Geelong because they have so many players who play in my position, and a lot of them were All-Australians last year," he told westernbulldogs.com.au.
"You're starting from back in the queue a little bit and that makes things pretty hard.
"Geelong footy club was always something that was pretty special to me because Dad played for them and I grew up supporting them.
"I had five great years down there, but at the same time, there comes a time to move on and I'm glad I've done that now and I'm really enjoying things up here."
The small defender is also thriving in his new environment according to coach Rodney Eade.
"There are some players that have put on some really good gains, and Tim's one," Eade said.
"He looks like a ball of muscle, doesn't he? It hasn't affected his running; his running is really good and I think his time trials are better than he's ever done before at Geelong.
"He's improved his fitness and he's certainly improved his strength. We've been really pleased with his attitude, he's really trained extremely well and, at this stage, I think he'll certainly play round one in the NAB Cup."
Callan said Eade had earmarked him for a role in the backline, with the potential to sneak "into the midfield and act as a tagger, if need be".
He was modest when speaking of the alleged increase in his former 81kg frame and is more pleased with the running ability he and his teammates have put together over the summer months.
"I said in the Herald Sun last week that I've put on five kilos, but I've put on maybe half a kilo at the most," he laughed.
"There's no real difference, but I've got my weight probably where I want it at the minute, so I'm pretty spot-on for what we want to do.
"It's been a pretty big preseason, and all the boys have added a bit of weight, as everyone knows.
"At the same time, they've kept all the running ability, so I think it's only going to be a pretty big plus when the games start."
Callan said he doesn't feel pressure to perform at his new club, but is intent on putting everything he has into both pre-season training and the year ahead.
"It was important to make an impression, but I don't really look at it as my last chance," he said.
"I've always done everything to the best of my abilities, with whatever I do in life, and I don't really leave anything to chance.
"I leave no stone unturned, so I suppose you could look at it that way but I'm really just going to go out there, have a crack and see what happens."
Callan saw Geelong's transformation from also-rans to premiers up close and he is enthusiastic about the Bulldogs' current situation.
"It's exciting to be here; I see a lot of similarities between where Geelong was 12 months ago and where the Western Bulldogs are now – probably not living up to expectations the previous year," he said.
"[Geelong] showed what can be achieved if you really put your head down and everyone bonds together and does everything properly."
