I believe they were, in fact, expecting Bulgaria to join the war on the Entente's side, or at least stay neutral. From what I've found it seems that the Entente repeatedly tried to offer parts of Macedonia to Bulgaria in exchange for military cooperation or neutrality; in exchange, Serbia would get more territory from Austria-Hungary. The Serbian government, of course, categorically refused to consider this (Wikipedia claims Serbia's insistence on keeping Macedonia even at the expense of Bosnia was based on a geopolitical theory that "held that the dominant position in the Balkan Peninsula would be held by the country that controlled the valleys of the rivers Morava and Vardar"; the citation for this statement seems at least a little questionable but it sounds plausible to me). Even as late as 1918 there are American-published predictions of post-war Europe that show Bulgaria gaining territory: the Rand McNally & Co. "Peace Map of Europe" shows Bulgaria picking up most of East Thrace and a little bit of Greek Macedonia, while "A Europe of liberty and peace for every nation" from F. A. Ferris & Company has Bulgaria give up their Aegean coastline and some western territory in exchange for Serbian Macedonia south of the Vardar plus the area around Monastir/Bitola, as well as a decent amount of (inland) Thrace and Greek Macedonia, and most of Dobruja.Why exactly is Bulgaria gaining territory? Were they expecting Bulgaria to be on the Allied side?
Also, why is Serbia losing land to the south if the Allies win?