That is actually not true!
Lets look at the equation of GDP.
GDP = C +G +(X-M) +I
Where
G: government spending,
C: consumer consumption
I: total investment
M: import while
X: export.
Since Tanzania imports (probably) more than Kenya Imports, mind you, Tanzania is a rich country so it affords to import more oil, more cars, more capital equipment, more cloths etc, that makes X to be larger than it exports. Mind you, we export less because we want to spend what we produce internally and there int enough production so far.
Sor the larger X when minused from smaller M, you get a a negative Nx that is net export. This will in overall make Tanzania's GDP look smaller, painting a gloomy picture of the economy.
That means, we we stop importing bulky things to our country, which your country don't do because you dont have enough money to spend and shop from abroad, that is why your GDP looks bigger, but that dont give a reality of the health of your economy.